Method and apparatus for encoding and decoding using selective information sharing between channels

ABSTRACT

Disclosed herein are a video decoding method and apparatus and a video encoding method and apparatus. Coding decision information of a representative channel of a target block is shared as coding decision information of a target channel of the target block, and decoding of the target block is performed using the coding decision information of the target channel. Since the coding decision information of the representative channel is shared with an additional channel, repeated signaling of identical coding decision information may be prevented. By means of this prevention, the efficiency of encoding and decoding of the target block or the like may be improved.

TECHNICAL FIELD

The following embodiments relate generally to a video decoding methodand apparatus and a video encoding method and apparatus, and moreparticularly, to a video decoding method and apparatus and a videoencoding method and apparatus that use sharing of selective informationbetween channels.

BACKGROUND ART

With the continuous development of the information and communicationindustries, broadcasting services supporting High-Definition (HD)resolution have been popularized all over the world. Through thispopularization, a large number of users have become accustomed tohigh-resolution and high-definition images and/or videos.

To satisfy users' demand for high definition, many institutions haveaccelerated the development of next-generation imaging devices. Users'interest in UHD TVs, having resolution that is more than four times ashigh as that of Full HD (FHD) TVs, as well as High-Definition TVs (HDTV)and FHD TVs, has increased. As interest therein has increased, imageencoding/decoding technology for images having higher resolution andhigher definition is continually required.

An image encoding/decoding apparatus and method may use inter-predictiontechnology, intra-prediction technology, entropy-coding technology, etc.so as to perform encoding/decoding on a high-resolution andhigh-definition image. Inter-prediction technology may be technology forpredicting the value of a pixel included in a target picture usingtemporally previous pictures and/or temporally subsequent pictures.Intra-prediction technology may be technology for predicting the valueof a pixel included in a target picture using information about pixelsin the target picture. Entropy-coding technology may be technology forassigning short code words to frequently occurring symbols and assigninglong code words to rarely occurring symbols.

Recently, demand for high-quality images, such as Ultra-High-Definition(UHD) images, which can provide high resolution, a further broadenedcolor space, and excellent image quality, has increased in variousapplication fields. As images trend toward higher-resolution andhigher-quality images, the amount of image data required in order toprovide images may be increased beyond that of existing image data. Withthe increase in the amount of image data, transmission costs and storagecosts are increased in the case where image data is transmitted throughcommunication media, such as wired/wireless broadband lines, or variousbroadcasting media, such as satellites, terrestrial waves, an InternetProtocol (IP) network, a wireless network, a cable, or a mobilecommunication network, or in the case where image data is stored invarious types of storage media, such as a Compact Disc (CD), a DigitalVersatile Disc (DVD), a Universal Serial Bus (USB) medium, and aHigh-Definition (HD)-DVD.

As high-resolution and high-quality images are used, high-efficiencyimage encoding/decoding technology is required in order to solveinevitable and more serious problems with image data and provide imageshaving higher resolution and higher image quality.

DISCLOSURE Technical Problem

An embodiment is intended to provide an encoding apparatus and methodand a decoding apparatus and method that use sharing of selectiveinformation between channels.

Technical Solution

In accordance with an aspect, there is provided a decoding method,including sharing coding decision information of a representativechannel of a target block as coding decision information of a targetchannel of the target block; and performing decoding on the target blockthat uses the coding decision information of the target channel.

The decoding method may further include receiving a bitstream includinginformation about the target block.

The information about the target block may include the coding decisioninformation of the representative channel.

The information about the target block may not include the codingdecision information of the target channel.

The coding decision information of the representative channel may betransform skip information indicating whether a transform is to beskipped.

The coding decision information of the representative channel mayindicate which transform is to be used for a transform block of achannel.

The coding decision information of the representative channel may beintra-coding decision information of the representative channel.

The representative channel and the target channel may be channels in aYCbCr color space.

The representative channel may be a luma channel.

The target channel may be a chroma channel.

The representative channel may be a color channel having a highestcorrelation with a luma signal.

The representative channel may be determined by an index indicating aselected representative channel in a bitstream.

The sharing may be performed when image properties of multiple channelsof the target block are similar to each other.

When an intra-prediction mode of a chroma channel of the target block isa direct mode, the image properties of the multiple channels may bedetermined to be similar to each other.

The sharing may be performed when cross-channel prediction is used.

Whether the cross-channel prediction is used may be derived based oninformation acquired from a bitstream.

The sharing may be performed when cross-channel prediction is used.

Whether the cross-channel prediction is used may be determined based onan intra-prediction mode of the target block.

When the intra-prediction mode of the target block is one of anINTRA_CCLM mode, an INTRA_MMLM mode, and an INTRA_MFLM mode,cross-channel prediction may be used.

Whether sharing is to be performed may be determined based on a size ofthe target block.

The coding decision information of the representative channel, amongmultiple channels of the target block, may be used for all of themultiple channels.

In accordance with another aspect, there is provided an encoding method,including determining coding decision information of a representativechannel of a target block; and performing encoding on the target blockthat uses the coding decision information of the representative channel,wherein the coding decision information of the representative channel isshared with an additional channel of the target block.

The encoding method may further include generating a bitstream includinginformation about the target block.

The information about the target block may include the coding decisioninformation of the representative channel.

The information about the target block may not include coding decisioninformation of the additional channel.

The representative channel and the additional channel may be channels ina YCbCr color space.

In accordance with a further aspect, there is provided acomputer-readable storage medium storing a bitstream for image decoding,the bitstream including information about a target block, wherein theinformation about the target block includes coding decision informationof a representative channel of the target block, wherein the codingdecision information of the representative channel of the target blockis used and shared as coding decision information of a target channel ofthe target block, and wherein decoding of the target block is performedusing the coding decision information of the target channel.

Advantageous Effects

There are provided an encoding apparatus and method and a decodingapparatus and method that use sharing of selective information betweenchannels.

DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating the configuration of anembodiment of an encoding apparatus to which the present disclosure isapplied;

FIG. 2 is a block diagram illustrating the configuration of anembodiment of a decoding apparatus to which the present disclosure isapplied;

FIG. 3 is a diagram schematically illustrating the partition structureof an image when the image is encoded and decoded;

FIG. 4 is a diagram illustrating the form of a Prediction Unit (PU) thata Coding Unit (CU) can include;

FIG. 5 is a diagram illustrating the form of a Transform Unit (TU) thatcan be included in a CU;

FIG. 6 illustrates splitting of a block according to an example;

FIG. 7 is a diagram for explaining an embodiment of an intra-predictionprocedure;

FIG. 8 is a diagram for explaining the locations of reference samplesused in an intra-prediction procedure;

FIG. 9 is a diagram for explaining an embodiment of an inter-predictionprocedure;

FIG. 10 illustrates spatial candidates according to an embodiment;

FIG. 11 illustrates the order of addition of motion information ofspatial candidates to a merge list according to an embodiment;

FIG. 12 illustrates a transform and quantization process according to anexample;

FIG. 13 illustrates diagonal scanning according to an example;

FIG. 14 illustrates horizontal scanning according to an example;

FIG. 15 illustrates vertical scanning according to an example;

FIG. 16 is a configuration diagram of an encoding apparatus according toan embodiment;

FIG. 17 is a configuration diagram of a decoding apparatus according toan embodiment;

FIG. 18 is a flowchart of a method for decoding coding decisioninformation according to an embodiment;

FIG. 19 is a flowchart of a decoding method for determining whether atransform is to be skipped according to an embodiment;

FIG. 20 is a flowchart of a decoding method for determining whether atransform is to be skipped with reference to an intra mode according toan embodiment;

FIG. 21 is a flowchart of a method for sharing transform selectioninformation according to an embodiment;

FIG. 22 illustrates a single-tree block partition structure according toan example;

FIG. 23 illustrates a dual-tree block partition structure according toan example;

FIG. 24 illustrates a scheme for specifying a corresponding block basedon a location in a corresponding region according to an example;

FIG. 25 illustrates a scheme for specifying a corresponding block basedon an area in a corresponding region according to an example;

FIG. 26 illustrates another scheme for specifying a corresponding blockbased on an area in a corresponding region according to an example;

FIG. 27 illustrates a scheme for specifying a corresponding block basedon the form of a block in a corresponding region according to anexample;

FIG. 28 illustrates another scheme for specifying a corresponding blockbased on the form of a block in a corresponding region according to anexample;

FIG. 29 illustrates a scheme for specifying a corresponding block basedon the aspect ratio of a block in a corresponding region according to anexample;

FIG. 30 illustrates another scheme for specifying a corresponding blockbased on the aspect ratio of a block in a corresponding region accordingto an example;

FIG. 31 illustrates a scheme for specifying a corresponding block basedon the encoding features of a block in a corresponding region accordingto an example;

FIG. 32 is a flowchart of an encoding method according to an embodiment;and

FIG. 33 is a flowchart of a decoding method according to an embodiment.

BEST MODE

The present invention may be variously changed, and may have variousembodiments, and specific embodiments will be described in detail belowwith reference to the attached drawings. However, it should beunderstood that those embodiments are not intended to limit the presentinvention to specific disclosure forms, and that they include allchanges, equivalents or modifications included in the spirit and scopeof the present invention.

Detailed descriptions of the following exemplary embodiments will bemade with reference to the attached drawings illustrating specificembodiments. These embodiments are described so that those havingordinary knowledge in the technical field to which the presentdisclosure pertains can easily practice the embodiments. It should benoted that the various embodiments are different from each other, but donot need to be mutually exclusive of each other. For example, specificshapes, structures, and characteristics described here may beimplemented as other embodiments without departing from the spirit andscope of the embodiments in relation to an embodiment. Further, itshould be understood that the locations or arrangement of individualcomponents in each disclosed embodiment can be changed without departingfrom the spirit and scope of the embodiments. Therefore, theaccompanying detailed description is not intended to restrict the scopeof the disclosure, and the scope of the exemplary embodiments is limitedonly by the accompanying claims, along with equivalents thereof, as longas they are appropriately described.

In the drawings, similar reference numerals are used to designate thesame or similar functions in various aspects. The shapes, sizes, etc. ofcomponents in the drawings may be exaggerated to make the descriptionclear.

Terms such as “first” and “second” may be used to describe variouscomponents, but the components are not restricted by the terms. Theterms are used only to distinguish one component from another component.For example, a first component may be named a second component withoutdeparting from the scope of the present specification. Likewise, asecond component may be named a first component. The terms “and/or” mayinclude combinations of a plurality of related described items or any ofa plurality of related described items.

It will be understood that when a component is referred to as being“connected” or “coupled” to another component, the two components may bedirectly connected or coupled to each other, or intervening componentsmay be present between the two components. It will be understood thatwhen a component is referred to as being “directly connected orcoupled”, no intervening components are present between the twocomponents.

Also, components described in the embodiments are independently shown inorder to indicate different characteristic functions, but this does notmean that each of the components is formed of a separate piece ofhardware or software. That is, the components are arranged and includedseparately for convenience of description. For example, at least two ofthe components may be integrated into a single component. Conversely,one component may be divided into multiple components. An embodimentinto which the components are integrated or an embodiment in which somecomponents are separated is included in the scope of the presentspecification as long as it does not depart from the essence of thepresent specification.

Further, it should be noted that, in the exemplary embodiments, anexpression describing that a component “comprises” a specific componentmeans that additional components may be included within the scope of thepractice or the technical spirit of exemplary embodiments, but does notpreclude the presence of components other than the specific component.

The terms used in the present specification are merely used to describespecific embodiments and are not intended to limit the presentinvention. A singular expression includes a plural expression unless adescription to the contrary is specifically pointed out in context. Inthe present specification, it should be understood that the terms suchas “include” or “have” are merely intended to indicate that features,numbers, steps, operations, components, parts, or combinations thereofare present, and are not intended to exclude the possibility that one ormore other features, numbers, steps, operations, components, parts, orcombinations thereof will be present or added.

Embodiments will be described in detail below with reference to theaccompanying drawings so that those having ordinary knowledge in thetechnical field to which the embodiments pertain can easily practice theembodiments. In the following description of the embodiments, detaileddescriptions of known functions or configurations which are deemed tomake the gist of the present specification obscure will be omitted.Further, the same reference numerals are used to designate the samecomponents throughout the drawings, and repeated descriptions of thesame components will be omitted.

Hereinafter, “image” may mean a single picture constituting a video, ormay mean the video itself. For example, “encoding and/or decoding of animage” may mean “encoding and/or decoding of a video”, and may also mean“encoding and/or decoding of any one of images constituting the video”.

Hereinafter, the terms “video” and “motion picture” may be used to havethe same meaning, and may be used interchangeably with each other.

Hereinafter, a target image may be an encoding target image, which isthe target to be encoded, and/or a decoding target image, which is thetarget to be decoded. Further, the target image may be an input imagethat is input to an encoding apparatus or an input image that is inputto a decoding apparatus.

Hereinafter, the terms “image”, “picture”, “frame”, and “screen” may beused to have the same meaning and may be used interchangeably with eachother.

Hereinafter, a target block may be an encoding target block, i.e. thetarget to be encoded and/or a decoding target block, i.e. the target tobe decoded. Further, the target block may be a current block, i.e. thetarget to be currently encoded and/or decoded. Here, the terms “targetblock” and “current block” may be used to have the same meaning, and maybe used interchangeably with each other.

Hereinafter, the terms “block” and “unit” may be used to have the samemeaning, and may be used interchangeably with each other. Alternatively,“block” may denote a specific unit.

Hereinafter, the terms “region” and “segment” may be usedinterchangeably with each other.

Hereinafter, a specific signal may be a signal indicating a specificblock. For example, the original signal may be a signal indicating atarget block. A prediction signal may be a signal indicating aprediction block. A residual signal may be a signal indicating aresidual block.

In the following embodiments, specific information, data, a flag, anelement, and an attribute may have their respective values. A value of“0” corresponding to each of the information, data, flag, element, andattribute may indicate a logical false or a first predefined value. Inother words, the value of “0”, false, logical false, and a firstpredefined value may be used interchangeably with each other. A value of“1” corresponding to each of the information, data, flag, element, andattribute may indicate a logical true or a second predefined value. Inother words, the value of “1”, true, logical true, and a secondpredefined value may be used interchangeably with each other.

When a variable such as i or j is used to indicate a row, a column, oran index, the value of i may be an integer of 0 or more or an integer of1 or more. In other words, in the embodiments, each of a row, a column,and an index may be counted from 0 or may be counted from 1.

Below, the terms to be used in embodiments will be described.

Encoder: An encoder denotes a device for performing encoding.

Decoder: A decoder denotes a device for performing decoding.

Unit: A unit may denote the unit of image encoding and decoding. Theterms “unit” and “block” may be used to have the same meaning, and maybe used interchangeably with each other.

-   -   “Unit” may be an M×N array of samples. M and N may be positive        integers, respectively. The term “unit” may generally mean a        two-dimensional (2D) array of samples.    -   In the encoding and decoding of an image, “unit” may be an area        generated by the partitioning of one image. In other words,        “unit” may be a region specified in one image. A single image        may be partitioned into multiple units. Alternatively, one image        may be partitioned into sub-parts, and the unit may denote each        partitioned sub-part when encoding or decoding is performed on        the partitioned sub-part.    -   In the encoding and decoding of an image, predefined processing        may be performed on each unit depending on the type of the unit.    -   Depending on functions, the unit types may be classified into a        macro unit, a Coding Unit (CU), a Prediction Unit (PU), a        residual unit, a Transform Unit (TU), etc. Alternatively,        depending on functions, the unit may denote a block, a        macroblock, a coding tree unit, a coding tree block, a coding        unit, a coding block, a prediction unit, a prediction block, a        residual unit, a residual block, a transform unit, a transform        block, etc.    -   The term “unit” may mean information including a luminance        (luma) component block, a chrominance (chroma) component block        corresponding thereto, and syntax elements for respective blocks        so that the unit is designated to be distinguished from a block.    -   The size and shape of a unit may be variously implemented.        Further, a unit may have any of various sizes and shapes. In        particular, the shapes of the unit may include not only a        square, but also a geometric figure that can be represented in        two dimensions (2D), such as a rectangle, a trapezoid, a        triangle, and a pentagon.    -   Further, unit information may include one or more of the type of        a unit, the size of a unit, the depth of a unit, the order of        encoding of a unit and the order of decoding of a unit, etc. For        example, the type of a unit may indicate one of a CU, a PU, a        residual unit and a TU.    -   One unit may be partitioned into sub-units, each having a        smaller size than that of the relevant unit.        -   Depth: A depth may denote the degree to which the unit is            partitioned. Further, the unit depth may indicate the level            at which the corresponding unit is present when units are            represented in a tree structure.        -   Unit partition information may include a depth indicating            the depth of a unit. A depth may indicate the number of            times the unit is partitioned and/or the degree to which the            unit is partitioned.        -   In a tree structure, it may be considered that the depth of            a root node is the smallest, and the depth of a leaf node is            the largest.        -   A single unit may be hierarchically partitioned into            multiple sub-units while having depth information based on a            tree structure. In other words, the unit and sub-units,            generated by partitioning the unit, may correspond to a node            and child nodes of the node, respectively. Each of the            partitioned sub-units may have a unit depth. Since the depth            indicates the number of times the unit is partitioned and/or            the degree to which the unit is partitioned, the partition            information of the sub-units may include information about            the sizes of the sub-units.        -   In a tree structure, the top node may correspond to the            initial node before partitioning. The top node may be            referred to as a “root node”. Further, the root node may            have a minimum depth value. Here, the top node may have a            depth of level ‘0’        -   A node having a depth of level ‘1’ may denote a unit            generated when the initial unit is partitioned once. A node            having a depth of level ‘2’ may denote a unit generated when            the initial unit is partitioned twice.        -   A leaf node having a depth of level ‘n’ may denote a unit            generated when the initial unit has been partitioned n            times.        -   The leaf node may be a bottom node, which cannot be            partitioned any further. The depth of the leaf node may be            the maximum level. For example, a predefined value for the            maximum level may be 3.        -   A QT depth may denote a depth for a quad-partitioning. A BT            depth may denote a depth for a binary-partitioning. A TT            depth may denote a depth for a ternary-partitioning.

Sample: A sample may be a base unit constituting a block. A sample maybe represented by values from 0 to 2^(Bd-)1 depending on the bit depth(Bd).

-   -   A sample may be a pixel or a pixel value.    -   Hereinafter, the terms “pixel” and “sample” may be used to have        the same meaning, and may be used interchangeably with each        other.

A Coding Tree Unit (CTU): A CTU may be composed of a single lumacomponent (Y) coding tree block and two chroma component (Cb, Cr) codingtree blocks related to the luma component coding tree block. Further, aCTU may mean information including the above blocks and a syntax elementfor each of the blocks.

-   -   Each coding tree unit (CTU) may be partitioned using one or more        partitioning methods, such as a quad tree (QT), a binary tree        (BT), and a ternary tree (TT) so as to configure sub-units, such        as a coding unit, a prediction unit, and a transform unit.        Further, each coding tree unit may be partitioned using a        multitype tree (MTT) using one or more partitioning methods.    -   “CTU” may be used as a term designating a pixel block, which is        a processing unit in an image-decoding and encoding process, as        in the case of partitioning of an input image.

Coding Tree Block (CTB): “CTB” may be used as a term designating any oneof a Y coding tree block, a Cb coding tree block, and a Cr coding treeblock.

Neighbor block: A neighbor block (or neighboring block) may mean a blockadjacent to a target block. A neighbor block may mean a reconstructedneighbor block.

Hereinafter, the terms “neighbor block” and “adjacent block” may be usedto have the same meaning and may be used interchangeably with eachother.

Spatial neighbor block; A spatial neighbor block may a block spatiallyadjacent to a target block. A neighbor block may include a spatialneighbor block.

-   -   The target block and the spatial neighbor block may be included        in a target picture.    -   The spatial neighbor block may mean a block, the boundary of        which is in contact with the target block, or a block located        within a predetermined distance from the target block.    -   The spatial neighbor block may mean a block adjacent to the        vertex of the target block. Here, the block adjacent to the        vertex of the target block may mean a block vertically adjacent        to a neighbor block which is horizontally adjacent to the target        block or a block horizontally adjacent to a neighbor block which        is vertically adjacent to the target block.

Temporal neighbor block: A temporal neighbor block may be a blocktemporally adjacent to a target block. A neighbor block may include atemporal neighbor block.

-   -   The temporal neighbor block may include a co-located block (col        block).    -   The col block may be a block in a previously reconstructed        co-located picture (col picture). The location of the col block        in the col-picture may correspond to the location of the target        block in a target picture. Alternatively, the location of the        col block in the col-picture may be equal to the location of the        target block in the target picture. The col picture may be a        picture included in a reference picture list.    -   The temporal neighbor block may be a block temporally adjacent        to a spatial neighbor block of a target block.

Prediction unit: A prediction unit may be a base unit for prediction,such as inter prediction, intra prediction, inter compensation, intracompensation, and motion compensation.

-   -   A single prediction unit may be divided into multiple partitions        having smaller sizes or sub-prediction units. The multiple        partitions may also be base units in the performance of        prediction or compensation. The partitions generated by dividing        the prediction unit may also be prediction units.

Prediction unit partition: A prediction unit partition may be the shapeinto which a prediction unit is divided.

Reconstructed neighboring unit: A reconstructed neighboring unit may bea unit which has already been decoded and reconstructed around a targetunit.

-   -   A reconstructed neighboring unit may be a unit that is spatially        adjacent to the target unit or that is temporally adjacent to        the target unit.    -   A reconstructed spatially neighboring unit may be a unit which        is included in a target picture and which has already been        reconstructed through encoding and/or decoding.    -   A reconstructed temporally neighboring unit may be a unit which        is included in a reference image and which has already been        reconstructed through encoding and/or decoding. The location of        the reconstructed temporally neighboring unit in the reference        image may be identical to that of the target unit in the target        picture, or may correspond to the location of the target unit in        the target picture.

Parameter set: A parameter set may be header information in thestructure of a bitstream. For example, a parameter set may include avideo parameter set, a sequence parameter set, a picture parameter set,an adaptation parameter set, etc.

Further, the parameter set may include slice header information and tileheader information.

Rate-distortion optimization: An encoding apparatus may userate-distortion optimization so as to provide high coding efficiency byutilizing combinations of the size of a coding unit (CU), a predictionmode, the size of a prediction unit (PU), motion information, and thesize of a transform unit (TU).

-   -   A rate-distortion optimization scheme may calculate        rate-distortion costs of respective combinations so as to select        an optimal combination from among the combinations. The        rate-distortion costs may be calculated using the following        Equation 1. Generally, a combination enabling the        rate-distortion cost to be minimized may be selected as the        optimal combination in the rate-distortion optimization scheme.

D+λ*R  [Equation 1]

-   -   D may denote distortion. D may be the mean of squares of        differences (i.e. mean square error) between original transform        coefficients and reconstructed transform coefficients in a        transform unit.    -   R may denote the rate, which may denote a bit rate using        related-context information.    -   λ denotes a Lagrangian multiplier. R may include not only coding        parameter information, such as a prediction mode, motion        information, and a coded block flag, but also bits generated due        to the encoding of transform coefficients.    -   An encoding apparatus may perform procedures, such as inter        prediction and/or intra prediction, transform, quantization,        entropy encoding, inverse quantization (dequantization), and        inverse transform so as to calculate precise D and R. These        procedures may greatly increase the complexity of the encoding        apparatus.    -   Bitstream: A bitstream may denote a stream of bits including        encoded image information.    -   Parameter set: A parameter set may be header information in the        structure of a bitstream.    -   The parameter set may include at least one of a video parameter        set, a sequence parameter set, a picture parameter set, and an        adaptation parameter set. Further, the parameter set may include        information about a slice header and information about a tile        header.

Parsing: Parsing may be the decision on the value of a syntax element,made by performing entropy decoding on a bitstream. Alternatively, theterm “parsing” may mean such entropy decoding itself.

Symbol: A symbol may be at least one of the syntax element, the codingparameter, and the transform coefficient of an encoding target unitand/or a decoding target unit. Further, a symbol may be the target ofentropy encoding or the result of entropy decoding.

Reference picture: A reference picture may be an image referred to by aunit so as to perform inter prediction or motion compensation.Alternatively, a reference picture may be an image including a referenceunit referred to by a target unit so as to perform inter prediction ormotion compensation.

Hereinafter, the terms “reference picture” and “reference image” may beused to have the same meaning, and may be used interchangeably with eachother.

Reference picture list: A reference picture list may be a list includingone or more reference images used for inter prediction or motioncompensation.

-   -   The types of a reference picture list may include List Combined        (LC), List 0 (L0), List 1 (L1), List 2 (L2), List 3 (L3), etc.    -   For inter prediction, one or more reference picture lists may be        used.

Inter-prediction indicator: An inter-prediction indicator may indicatethe inter-prediction direction for a target unit. Inter prediction maybe one of unidirectional prediction and bidirectional prediction.Alternatively, the inter-prediction indicator may denote the number ofreference images used to generate a prediction unit of a target unit.Alternatively, the inter-prediction indicator may denote the number ofprediction blocks used for inter prediction or motion compensation of atarget unit.

Reference picture index: A reference picture index may be an indexindicating a specific reference image in a reference picture list.

Motion vector (MV): A motion vector may be a 2D vector used for interprediction or motion compensation. A motion vector may mean an offsetbetween a target image and a reference image.

-   -   For example, a MV may be represented in a form such as (mv_(x),        mv_(y)). mv_(x) may indicate a horizontal component, and mv_(y)        may indicate a vertical component.    -   Search range: A search range may be a 2D area in which a search        for a MV is performed during inter prediction. For example, the        size of the search range may be M×N. M and N may be respective        positive integers.

Motion vector candidate: A motion vector candidate may be a block thatis a prediction candidate or the motion vector of the block that is aprediction candidate when a motion vector is predicted.

-   -   A motion vector candidate may be included in a motion vector        candidate list.

Motion vector candidate list: A motion vector candidate list may be alist configured using one or more motion vector candidates.

Motion vector candidate index: A motion vector candidate index may be anindicator for indicating a motion vector candidate in the motion vectorcandidate list. Alternatively, a motion vector candidate index may bethe index of a motion vector predictor.

Motion information: Motion information may be information including atleast one of a reference picture list, a reference image, a motionvector candidate, a motion vector candidate index, a merge candidate,and a merge index, as well as a motion vector, a reference pictureindex, and an inter-prediction indicator.

Merge candidate list: A merge candidate list may be a list configuredusing merge candidates.

Merge candidate: A merge candidate may be a spatial merge candidate, atemporal merge candidate, a combined merge candidate, a combinedbi-prediction merge candidate, a zero-merge candidate, etc. A mergecandidate may include motion information such as prediction typeinformation, a reference picture index for each list, and a motionvector.

Merge index: A merge index may be an indicator for indicating a mergecandidate in a merge candidate list.

-   -   A merge index may indicate a reconstructed unit used to derive a        merge candidate between a reconstructed unit spatially adjacent        to a target unit and a reconstructed unit temporally adjacent to        the target unit.    -   A merge index may indicate at least one of pieces of motion        information of a merge candidate.

Transform unit: A transform unit may be the base unit of residual signalencoding and/or residual signal decoding, such as transform, inversetransform, quantization, dequantization, transform coefficient encoding,and transform coefficient decoding. A single transform unit may bepartitioned into multiple transform units having smaller sizes.

Scaling: Scaling may denote a procedure for multiplying a factor by atransform coefficient level.

-   -   As a result of scaling of the transform coefficient level, a        transform coefficient may be generated. Scaling may also be        referred to as “dequantization”.

Quantization Parameter (QP): A quantization parameter may be a valueused to generate a transform coefficient level for a transformcoefficient in quantization. Alternatively, a quantization parameter mayalso be a value used to generate a transform coefficient by scaling thetransform coefficient level in dequantization. Alternatively, aquantization parameter may be a value mapped to a quantization stepsize.

Delta quantization parameter: A delta quantization parameter is adifferential value between a predicted quantization parameter and thequantization parameter of a target unit.

Scan: Scan may denote a method for aligning the order of coefficients ina unit, a block or a matrix. For example, a method for aligning a 2Darray in the form of a one-dimensional (1D) array may be referred to asa “scan”. Alternatively, a method for aligning a 1D array in the form ofa 2D array may also be referred to as a “scan” or an “inverse scan”.

Transform coefficient: A transform coefficient may be a coefficientvalue generated as an encoding apparatus performs a transform.Alternatively, the transform coefficient may be a coefficient valuegenerated as a decoding apparatus performs at least one of entropydecoding and dequantization.

-   -   A quantized level or a quantized transform coefficient level        generated by applying quantization to a transform coefficient or        a residual signal may also be included in the meaning of the        term “transform coefficient”.

Quantized level: A quantized level may be a value generated as theencoding apparatus performs quantization on a transform coefficient or aresidual signal. Alternatively, the quantized level may be a value thatis the target of dequantization as the decoding apparatus performsdequantization.

-   -   A quantized transform coefficient level, which is the result of        transform and quantization, may also be included in the meaning        of a quantized level.

Non-zero transform coefficient: A non-zero transform coefficient may bea transform coefficient having a value other than 0 or a transformcoefficient level having a value other than 0. Alternatively, a non-zerotransform coefficient may be a transform coefficient, the magnitude ofthe value of which is not 0, or a transform coefficient level, themagnitude of the value of which is not 0.

Quantization matrix: A quantization matrix may be a matrix used in aquantization procedure or a dequantization procedure so as to improvethe subjective image quality or objective image quality of an image. Aquantization matrix may also be referred to as a “scaling list”.

Quantization matrix coefficient: A quantization matrix coefficient maybe each element in a quantization matrix. A quantization matrixcoefficient may also be referred to as a “matrix coefficient”.

Default matrix: A default matrix may be a quantization matrix predefinedby the encoding apparatus and the decoding apparatus.

Non-default matrix: A non-default matrix may be a quantization matrixthat is not predefined by the encoding apparatus and the decodingapparatus. The non-default matrix may be signaled by the encodingapparatus to the decoding apparatus.

Most Probable Mode (MPM): An MPM may denote an intra-prediction modehaving a high probability of being used for intra prediction for atarget block.

An encoding apparatus and a decoding apparatus may determine one or moreMPMs based on coding parameters related to the target block and theattributes of entities related to the target block.

The encoding apparatus and the decoding apparatus may determine one ormore MPMs based on the intra-prediction mode of a reference block. Thereference block may include multiple reference blocks. The multiplereference blocks may include spatial neighbor blocks adjacent to theleft of the target block and spatial neighbor blocks adjacent to the topof the target block. In other words, depending on which intra-predictionmodes have been used for the reference blocks, one or more differentMPMs may be determined.

The one or more MPMs may be determined in the same manner both in theencoding apparatus and in the decoding apparatus. That is, the encodingapparatus and the decoding apparatus may share the same MPM listincluding one or more MPMs.

MPM list: An MPM list may be a list including one or more MPMs. Thenumber of the one or more MPMs in the MPM list may be defined inadvance.

MPM indicator: An MPM indicator may indicate an MPM to be used for intraprediction for a target block among one or more MPMs in the MPM list.For example, the MPM indicator may be an index for the MPM list.

Since the MPM list is determined in the same manner both in the encodingapparatus and in the decoding apparatus, there may be no need totransmit the MPM list itself from the encoding apparatus to the decodingapparatus.

The MPM indicator may be signaled from the encoding apparatus to thedecoding apparatus. As the MPM indicator is signaled, the decodingapparatus may determine the MPM to be used for intra prediction for thetarget block among the MPMs in the MPM list.

MPM use indicator: An MPM use indicator may indicate whether an MPMusage mode is to be used for prediction for a target block. The MPMusage mode may be a mode in which the MPM to be used for intraprediction for the target block is determined using the MPM list.

The MPM usage indicator may be signaled from the encoding apparatus tothe decoding apparatus.

Signaling: “signaling” may denote that information is transferred froman encoding apparatus to a decoding apparatus. Alternatively,“signaling” may mean information is included in in a bitstream or arecoding medium. Information signaled by an encoding apparatus may beused by a decoding apparatus.

FIG. 1 is a block diagram illustrating the configuration of anembodiment of an encoding apparatus to which the present disclosure isapplied.

An encoding apparatus 100 may be an encoder, a video encoding apparatusor an image encoding apparatus. A video may include one or more images(pictures). The encoding apparatus 100 may sequentially encode one ormore images of the video.

Referring to FIG. 1, the encoding apparatus 100 includes aninter-prediction unit 110, an intra-prediction unit 120, a switch 115, asubtractor 125, a transform unit 130, a quantization unit 140, anentropy encoding unit 150, a dequantization (inverse quantization) unit160, an inverse transform unit 170, an adder 175, a filter unit 180, anda reference picture buffer 190.

The encoding apparatus 100 may perform encoding on a target image usingan intra mode and/or an inter mode.

Further, the encoding apparatus 100 may generate a bitstream, includinginformation about encoding, via encoding on the target image, and mayoutput the generated bitstream. The generated bitstream may be stored ina computer-readable storage medium and may be streamed through awired/wireless transmission medium.

When the intra mode is used as a prediction mode, the switch 115 mayswitch to the intra mode. When the inter mode is used as a predictionmode, the switch 115 may switch to the inter mode.

The encoding apparatus 100 may generate a prediction block of a targetblock. Further, after the prediction block has been generated, theencoding apparatus 100 may encode a residual between the target blockand the prediction block.

When the prediction mode is the intra mode, the intra-prediction unit120 may use pixels of previously encoded/decoded neighboring blocksaround the target block as reference samples. The intra-prediction unit120 may perform spatial prediction on the target block using thereference samples, and may generate prediction samples for the targetblock via spatial prediction.

The inter-prediction unit 110 may include a motion prediction unit and amotion compensation unit.

When the prediction mode is an inter mode, the motion prediction unitmay search a reference image for the area most closely matching thetarget block in a motion prediction procedure, and may derive a motionvector for the target block and the found area based on the found area.

The reference image may be stored in the reference picture buffer 190.More specifically, the reference image may be stored in the referencepicture buffer 190 when the encoding and/or decoding of the referenceimage have been processed.

The motion compensation unit may generate a prediction block for thetarget block by performing motion compensation using a motion vector.Here, the motion vector may be a two-dimensional (2D) vector used forinter-prediction. Further, the motion vector may indicate an offsetbetween the target image and the reference image.

The motion prediction unit and the motion compensation unit may generatea prediction block by applying an interpolation filter to a partial areaof a reference image when the motion vector has a value other than aninteger. In order to perform inter prediction or motion compensation, itmay be determined which one of a skip mode, a merge mode, an advancedmotion vector prediction (AMVP) mode, and a current picture referencemode corresponds to a method for predicting the motion of a PU includedin a CU, based on the CU, and compensating for the motion, and interprediction or motion compensation may be performed depending on themode.

The subtractor 125 may generate a residual block, which is thedifferential between the target block and the prediction block. Aresidual block may also be referred to as a “residual signal”.

The residual signal may be the difference between an original signal anda prediction signal. Alternatively, the residual signal may be a signalgenerated by transforming or quantizing the difference between anoriginal signal and a prediction signal or by transforming andquantizing the difference. A residual block may be a residual signal fora block unit.

The transform unit 130 may generate a transform coefficient bytransforming the residual block, and may output the generated transformcoefficient. Here, the transform coefficient may be a coefficient valuegenerated by transforming the residual block.

The transform unit 130 may use one of multiple predefined transformmethods when performing a transform.

The multiple predefined transform methods may include a Discrete CosineTransform (DCT), a Discrete Sine Transform (DST), a Karhunen-LoeveTransform (KLT), etc.

The transform method used to transform a residual block may bedetermined depending on at least one of coding parameters for a targetblock and/or a neighboring block. For example, the transform method maybe determined based on at least one of an inter-prediction mode for aPU, an intra-prediction mode for a PU, the size of a TU, and the shapeof a TU. Alternatively, transformation information indicating thetransform method may be signaled from the encoding apparatus 100 to thedecoding apparatus 200.

When a transform skip mode is used, the transform unit 130 may omittransforming the residual block.

By applying quantization to the transform coefficient, a quantizedtransform coefficient level or a quantized level may be generated.Hereinafter, in the embodiments, each of the quantized transformcoefficient level and the quantized level may also be referred to as a‘transform coefficient’.

The quantization unit 140 may generate a quantized transform coefficientlevel or a quantized level by quantizing the transform coefficientdepending on quantization parameters. The quantization unit 140 mayoutput the quantized transform coefficient level or the quantized levelthat is generated. In this case, the quantization unit 140 may quantizethe transform coefficient using a quantization matrix.

The entropy encoding unit 150 may generate a bitstream by performingprobability distribution-based entropy encoding based on values,calculated by the quantization unit 140, and/or coding parameter values,calculated in the encoding procedure. The entropy encoding unit 150 mayoutput the generated bitstream.

The entropy encoding unit 150 may perform entropy encoding oninformation about the pixels of the image and information required todecode the image. For example, the information required to decode theimage may include syntax elements or the like.

When entropy encoding is applied, fewer bits may be assigned to morefrequently occurring symbols, and more bits may be assigned to rarelyoccurring symbols. As symbols are represented by means of thisassignment, the size of a bit string for target symbols to be encodedmay be reduced. Therefore, the compression performance of video encodingmay be improved through entropy encoding.

Further, for entropy encoding, the entropy encoding unit 150 may use acoding method such as exponential Golomb, Context-Adaptive VariableLength Coding (CAVLC), or Context-Adaptive Binary Arithmetic Coding(CABAC). For example, the entropy encoding unit 150 may perform entropyencoding using a Variable Length Coding/Code (VLC) table. For example,the entropy encoding unit 150 may derive a binarization method for atarget symbol. Further, the entropy encoding unit 150 may derive aprobability model for a target symbol/bin. The entropy encoding unit 150may perform arithmetic coding using the derived binarization method, aprobability model, and a context model.

The entropy encoding unit 150 may transform the coefficient of the formof a 2D block into the form of a 1D vector through a transformcoefficient scanning method so as to encode a quantized transformcoefficient level.

The coding parameters may be information required for encoding and/ordecoding. The coding parameters may include information encoded by theencoding apparatus 100 and transferred from the encoding apparatus 100to a decoding apparatus, and may also include information that may bederived in the encoding or decoding procedure. For example, informationtransferred to the decoding apparatus may include syntax elements.

The coding parameters may include not only information (or a flag or anindex), such as a syntax element, which is encoded by the encodingapparatus and is signaled by the encoding apparatus to the decodingapparatus, but also information derived in an encoding or decodingprocess. Further, the coding parameters may include information requiredso as to encode or decode images. For example, the coding parameters mayinclude at least one value, combinations or statistics of the size of aunit/block, the depth of a unit/block, partition information of aunit/block, the partition structure of a unit/block, informationindicating whether a unit/block is partitioned in a quad-tree structure,information indicating whether a unit/block is partitioned in a binarytree structure, the partitioning direction of a binary tree structure(horizontal direction or vertical direction), the partitioning form of abinary tree structure (symmetrical partitioning or asymmetricalpartitioning), information indicating whether a unit/block ispartitioned in a ternary tree structure, the partitioning direction of aternary tree structure (horizontal direction or vertical direction), thepartitioning form of a ternary tree structure (symmetrical partitioningor asymmetrical partitioning, etc.), information indicating whether aunit/block is partitioned in a complex tree structure, a combination anda direction (horizontal direction or vertical direction, etc.) of apartitioning of the complex tree structure, a prediction scheme (intraprediction or inter prediction), an intra-prediction mode/direction, areference sample filtering method, a prediction block filtering method,a prediction block boundary filtering method, a filter tap forfiltering, a filter coefficient for filtering, an inter-prediction mode,motion information, a motion vector, a reference picture index, aninter-prediction direction, an inter-prediction indicator, a referencepicture list, a reference image, a motion vector predictor, a motionvector prediction candidate, a motion vector candidate list, informationindicating whether a merge mode is used, a merge candidate, a mergecandidate list, information indicating whether a skip mode is used, thetype of an interpolation filter, the tap of an interpolation filter, thefilter coefficient of an interpolation filter, the magnitude of a motionvector, accuracy of motion vector representation, a transform type, atransform size, information indicating whether a primary transform isused, information indicating whether an additional (secondary) transformis used, first transform selection information (or a first transformindex), secondary transform selection information (or a secondarytransform index), information indicating the presence or absence of aresidual signal, a coded block pattern, a coded block flag, aquantization parameter, a quantization matrix, information about anintra-loop filter, information indicating whether an intra-loop filteris applied, the coefficient of an intra-loop filter, the tap of anintra-loop filter, the shape/form of an intra-loop filter, informationindicating whether a deblocking filter is applied, the coefficient of adeblocking filter, the tap of a deblocking filter, deblocking filterstrength, the shape/form of a deblocking filter, information indicatingwhether an adaptive sample offset is applied, the value of an adaptivesample offset, the category of an adaptive sample offset, the type of anadaptive sample offset, information indicating whether an adaptivein-loop filter is applied, the coefficient of an adaptive in-loopfilter, the tap of an adaptive in-loop filter, the shape/form of anadaptive in-loop filter, a binarization/inverse binarization method, acontext model, a context model decision method, a context model updatemethod, information indicating whether a regular mode is performed,information whether a bypass mode is performed, a context bin, a bypassbin, a transform coefficient, a transform coefficient level, a transformcoefficient level scanning method, an image display/output order, sliceidentification information, a slice type, slice partition information,tile identification information, a tile type, tile partitioninformation, a picture type, bit depth, information about a luma signal,and information about a chroma signal. The prediction scheme may denoteone prediction mode of an intra prediction mode and an inter predictionmode.

The first transform selection information may indicate a first transformwhich is applied to a target block.

The second transform selection information may indicate a secondtransform which is applied to a target block.

The residual signal may denote the difference between the originalsignal and a prediction signal. Alternatively, the residual signal maybe a signal generated by transforming the difference between theoriginal signal and the prediction signal. Alternatively, the residualsignal may be a signal generated by transforming and quantizing thedifference between the original signal and the prediction signal. Aresidual block may be the residual signal for a block.

Here, signaling a flag or an index may mean that the encoding apparatus100 includes an entropy-encoded flag or an entropy-encoded index,generated by performing entropy encoding on the flag or index, in abitstream, and that the decoding apparatus 200 acquires a flag or anindex by performing entropy decoding on the entropy-encoded flag or theentropy-encoded index, extracted from the bitstream.

Since the encoding apparatus 100 performs encoding via inter prediction,the encoded target image may be used as a reference image for additionalimage(s) to be subsequently processed. Therefore, the encoding apparatus100 may reconstruct or decode the encoded target image and store thereconstructed or decoded image as a reference image in the referencepicture buffer 190. For decoding, dequantization and inverse transformon the encoded target image may be processed.

The quantized level may be inversely quantized by the dequantizationunit 160, and may be inversely transformed by the inverse transform unit170. The coefficient that has been inversely quantized and/or inverselytransformed may be added to the prediction block by the adder 175. Theinversely quantized and/or inversely transformed coefficient and theprediction block are added, and then a reconstructed block may begenerated. Here, the inversely quantized and/or inversely transformedcoefficient may denote a coefficient on which one or more ofdequantization and inverse transform are performed, and may also denotea reconstructed residual block.

The reconstructed block may be subjected to filtering through the filterunit 180. The filter unit 180 may apply one or more of a deblockingfilter, a Sample Adaptive Offset (SAO) filter, an Adaptive Loop Filter(ALF) and a Non Local Filter (NLF) to the reconstructed block or areconstructed picture. The filter unit 180 may also be referred to as an“in-loop filter”.

The deblocking filter may eliminate block distortion occurring at theboundaries between blocks. In order to determine whether to apply thedeblocking filter, the number of columns or rows which are included in ablock and which include pixel(s) based on which it is determined whetherto apply the deblocking filter to a target block may be decided on.

When the deblocking filter is applied to the target block, the appliedfilter may differ depending on the strength of the required deblockingfiltering. In other words, among different filters, a filter decided onin consideration of the strength of deblocking filtering may be appliedto the target block. When a deblocking filter is applied to a targetblock, a filter corresponding to any one of a strong filter and a weakfilter may be applied to the target block depending on the strength ofrequired deblocking filtering.

Also, when vertical filtering and horizontal filtering are performed onthe target block, the horizontal filtering and the vertical filteringmay be processed in parallel.

The SAO may add a suitable offset to the values of pixels to compensatefor coding error. The SAO may perform, for the image to which deblockingis applied, correction that uses an offset in the difference between anoriginal image and the image to which deblocking is applied, on a pixelbasis. To perform an offset correction for an image, a method fordividing the pixels included in the image into a certain number ofregions, determining a region to which an offset is to be applied, amongthe divided regions, and applying an offset to the determined region maybe used, and a method for applying an offset in consideration of edgeinformation of each pixel may also be used.

The ALF may perform filtering based on a value obtained by comparing areconstructed image with an original image. After pixels included in animage have been divided into a predetermined number of groups, filtersto be applied to each group may be determined, and filtering may bedifferentially performed for respective groups. For a luma signal,information related to whether to apply an adaptive loop filter may besignaled for each CU. The shapes and filter coefficients of ALFs to beapplied to respective blocks may differ for respective blocks.Alternatively, regardless of the features of a block, an ALF having afixed form may be applied to the block.

A non-local filter may perform filtering based on reconstructed blocks,similar to a target block. A region similar to the target block may beselected from a reconstructed picture, and filtering of the target blockmay be performed using the statistical properties of the selectedsimilar region. Information about whether to apply a non-local filtermay be signaled for a Coding Unit (CU). Also, the shapes and filtercoefficients of the non-local filter to be applied to blocks may differdepending on the blocks.

The reconstructed block or the reconstructed image subjected tofiltering through the filter unit 180 may be stored in the referencepicture buffer 190. The reconstructed block subjected to filteringthrough the filter unit 180 may be a part of a reference picture. Inother words, the reference picture may be a reconstructed picturecomposed of reconstructed blocks subjected to filtering through thefilter unit 180. The stored reference picture may be subsequently usedfor inter prediction.

FIG. 2 is a block diagram illustrating the configuration of anembodiment of a decoding apparatus to which the present disclosure isapplied.

A decoding apparatus 200 may be a decoder, a video decoding apparatus oran image decoding apparatus.

Referring to FIG. 2, the decoding apparatus 200 may include an entropydecoding unit 210, a dequantization (inverse quantization) unit 220, aninverse transform unit 230, an intra-prediction unit 240, aninter-prediction unit 250, a switch 245 an adder 255, a filter unit 260,and a reference picture buffer 270.

The decoding apparatus 200 may receive a bitstream output from theencoding apparatus 100. The decoding apparatus 200 may receive abitstream stored in a computer-readable storage medium, and may receivea bitstream that is streamed through a wired/wireless transmissionmedium.

The decoding apparatus 200 may perform decoding on the bitstream in anintra mode and/or an inter mode. Further, the decoding apparatus 200 maygenerate a reconstructed image or a decoded image via decoding, and mayoutput the reconstructed image or decoded image.

For example, switching to an intra mode or an inter mode based on theprediction mode used for decoding may be performed by the switch 245.When the prediction mode used for decoding is an intra mode, the switch245 may be operated to switch to the intra mode. When the predictionmode used for decoding is an inter mode, the switch 245 may be operatedto switch to the inter mode.

The decoding apparatus 200 may acquire a reconstructed residual block bydecoding the input bitstream, and may generate a prediction block. Whenthe reconstructed residual block and the prediction block are acquired,the decoding apparatus 200 may generate a reconstructed block, which isthe target to be decoded, by adding the reconstructed residual block tothe prediction block.

The entropy decoding unit 210 may generate symbols by performing entropydecoding on the bitstream based on the probability distribution of abitstream. The generated symbols may include quantized transformcoefficient level-format symbols. Here, the entropy decoding method maybe similar to the above-described entropy encoding method. That is, theentropy decoding method may be the reverse procedure of theabove-described entropy encoding method.

The entropy decoding unit 210 may change a coefficient having aone-dimensional (1D) vector form to a 2D block shape through a transformcoefficient scanning method in order to decode a quantized transformcoefficient level.

For example, the coefficients of the block may be changed to 2D blockshapes by scanning the block coefficients using up-right diagonalscanning. Alternatively, which one of up-right diagonal scanning,vertical scanning, and horizontal scanning is to be used may bedetermined depending on the size and/or the intra-prediction mode of thecorresponding block.

The quantized coefficient may be inversely quantized by thedequantization unit 220. The dequantization unit 220 may generate aninversely quantized coefficient by performing dequantization on thequantized coefficient. Further, the inversely quantized coefficient maybe inversely transformed by the inverse transform unit 230. The inversetransform unit 230 may generate a reconstructed residual block byperforming an inverse transform on the inversely quantized coefficient.As a result of performing dequantization and the inverse transform onthe quantized coefficient, the reconstructed residual block may begenerated. Here, the dequantization unit 220 may apply a quantizationmatrix to the quantized coefficient when generating the reconstructedresidual block.

When the intra mode is used, the intra-prediction unit 240 may generatea prediction block by performing spatial prediction that uses the pixelvalues of previously decoded neighboring blocks around a target block.

The inter-prediction unit 250 may include a motion compensation unit.Alternatively, the inter-prediction unit 250 may be designated as a“motion compensation unit”.

When the inter mode is used, the motion compensation unit may generate aprediction block by performing motion compensation that uses a motionvector and a reference image stored in the reference picture buffer 270.

The motion compensation unit may apply an interpolation filter to apartial area of the reference image when the motion vector has a valueother than an integer, and may generate a prediction block using thereference image to which the interpolation filter is applied. In orderto perform motion compensation, the motion compensation unit maydetermine which one of a skip mode, a merge mode, an Advanced MotionVector Prediction (AMVP) mode, and a current picture reference modecorresponds to the motion compensation method used for a PU included ina CU, based on the CU, and may perform motion compensation depending onthe determined mode.

The reconstructed residual block and the prediction block may be addedto each other by the adder 255. The adder 255 may generate areconstructed block by adding the reconstructed residual block to theprediction block.

The reconstructed block may be subjected to filtering through the filterunit 260. The filter unit 260 may apply at least one of a deblockingfilter, an SAO filter, an ALF, and a NLF to the reconstructed block orthe reconstructed image. The reconstructed image may be a pictureincluding the reconstructed block.

The reconstructed image subjected to filtering may be outputted by theencoding apparatus 100, and may be used by the encoding apparatus.

The reconstructed image subjected to filtering through the filter unit260 may be stored as a reference picture in the reference picture buffer270. The reconstructed block subjected to filtering through the filterunit 260 may be a part of the reference picture. In other words, thereference picture may be an image composed of reconstructed blockssubjected to filtering through the filter unit 260. The stored referencepicture may be subsequently used for inter prediction.

FIG. 3 is a diagram schematically illustrating the partition structureof an image when the image is encoded and decoded.

FIG. 3 may schematically illustrate an example in which a single unit ispartitioned into multiple sub-units.

In order to efficiently partition the image, a Coding Unit (CU) may beused in encoding and decoding. The term “unit” may be used tocollectively designate 1) a block including image samples and 2) asyntax element. For example, the “partitioning of a unit” may mean the“partitioning of a block corresponding to a unit”.

A CU may be used as a base unit for image encoding/decoding. A CU may beused as a unit to which one mode selected from an intra mode and aninter mode in image encoding/decoding is applied. In other words, inimage encoding/decoding, which one of an intra mode and an inter mode isto be applied to each CU may be determined.

Further, a CU may be a base unit in prediction, transform, quantization,inverse transform, dequantization, and encoding/decoding of transformcoefficients.

Referring to FIG. 3, an image 200 may be sequentially partitioned intounits corresponding to a Largest Coding Unit (LCU), and a partitionstructure may be determined for each LCU. Here, the LCU may be used tohave the same meaning as a Coding Tree Unit (CTU).

The partitioning of a unit may mean the partitioning of a blockcorresponding to the unit. Block partition information may include depthinformation about the depth of a unit. The depth information mayindicate the number of times the unit is partitioned and/or the degreeto which the unit is partitioned. A single unit may be hierarchicallypartitioned into sub-units while having depth information based on atree structure. Each of partitioned sub-units may have depthinformation. The depth information may be information indicating thesize of a CU. The depth information may be stored for each CU.

Each CU may have depth information. When the CU is partitioned, CUsresulting from partitioning may have a depth increased from the depth ofthe partitioned CU by 1.

The partition structure may mean the distribution of Coding Units (CUs)to efficiently encode the image in an LCU 310. Such a distribution maybe determined depending on whether a single CU is to be partitioned intomultiple CUs. The number of CUs generated by partitioning may be apositive integer of 2 or more, including 2, 3, 4, 8, 16, etc. Thehorizontal size and the vertical size of each of CUs generated by thepartitioning may be less than the horizontal size and the vertical sizeof a CU before being partitioned, depending on the number of CUsgenerated by partitioning.

Each partitioned CU may be recursively partitioned into four CUs in thesame way. Via the recursive partitioning, at least one of the horizontalsize and the vertical size of each partitioned CU may be reducedcompared to at least one of the horizontal size and the vertical size ofthe CU before being partitioned.

The partitioning of a CU may be recursively performed up to a predefineddepth or a predefined size. For example, the depth of a CU may have avalue ranging from 0 to 3. The size of the CU may range from a size of64×64 to a size of 8×8 depending on the depth of the CU.

For example, the depth of an LCU may be 0, and the depth of a SmallestCoding Unit (SCU) may be a predefined maximum depth. Here, as describedabove, the LCU may be the CU having the maximum coding unit size, andthe SCU may be the CU having the minimum coding unit size.

Partitioning may start at the LCU 310, and the depth of a CU may beincreased by 1 whenever the horizontal and/or vertical sizes of the CUare reduced by partitioning.

For example, for respective depths, a CU that is not partitioned mayhave a size of 2N×2N. Further, in the case of a CU that is partitioned,a CU having a size of 2N×2N may be partitioned into four CUs, eachhaving a size of N×N. The value of N may be halved whenever the depth isincreased by 1.

Referring to FIG. 3, an LCU having a depth of 0 may have 64×64 pixels or64×64 blocks. 0 may be a minimum depth. An SCU having a depth of 3 mayhave 8×8 pixels or 8×8 blocks. 3 may be a maximum depth. Here, a CUhaving 64×64 blocks, which is the LCU, may be represented by a depth of0. A CU having 32×32 blocks may be represented by a depth of 1. A CUhaving 16×16 blocks may be represented by a depth of 2. A CU having 8×8blocks, which is the SCU, may be represented by a depth of 3.

Information about whether the corresponding CU is partitioned may berepresented by the partition information of the CU. The partitioninformation may be 1-bit information. All CUs except the SCU may includepartition information. For example, the value of the partitioninformation of a CU that is not partitioned may be 0. The value of thepartition information of a CU that is partitioned may be 1.

For example, when a single CU is partitioned into four CUs, thehorizontal size and vertical size of each of four CUs generated bypartitioning may be half the horizontal size and the vertical size ofthe CU before being partitioned. When a CU having a 32×32 size ispartitioned into four CUs, the size of each of four partitioned CUs maybe 16×16. When a single CU is partitioned into four CUs, it may beconsidered that the CU has been partitioned in a quad-tree structure.

For example, when a single CU is partitioned into two CUs, thehorizontal size or the vertical size of each of two CUs generated bypartitioning may be half the horizontal size or the vertical size of theCU before being partitioned. When a CU having a 32×32 size is verticallypartitioned into two CUs, the size of each of two partitioned CUs may be16×32. When a CU having a 32×32 size is horizontally partitioned intotwo CUs, the size of each of two partitioned CUs may be 32×16. When asingle CU is partitioned into two CUs, it may be considered that the CUhas been partitioned in a binary-tree structure.

Both of quad-tree partitioning and binary-tree partitioning are appliedto the LCU 310 of FIG. 3.

In the encoding apparatus 100, a Coding Tree Unit (CTU) having a size of64×64 may be partitioned into multiple smaller CUs by a recursivequad-tree structure. A single CU may be partitioned into four CUs havingthe same size. Each CU may be recursively partitioned, and may have aquad-tree structure.

By the recursive partitioning of a CU, an optimal partitioning methodthat incurs a minimum rate-distortion cost may be selected.

FIG. 4 is a diagram illustrating the form of a Prediction Unit (PU) thata Coding Unit (CU) can include.

When, among CUs partitioned from an LCU, a CU, which is not partitionedany further, may be divided into one or more Prediction Units (PUs).Such division is also referred to as “partitioning”.

A PU may be a base unit for prediction. A PU may be encoded and decodedin any one of a skip mode, an inter mode, and an intra mode. A PU may bepartitioned into various shapes depending on respective modes. Forexample, the target block, described above with reference to FIG. 1, andthe target block, described above with reference to FIG. 2, may each bea PU.

A CU may not be split into PUs. When the CU is not split into PUs, thesize of the CU and the size of a PU may be equal to each other.

In a skip mode, partitioning may not be present in a CU. In the skipmode, a 2N×2N mode 410, in which the sizes of a PU and a CU areidentical to each other, may be supported without partitioning.

In an inter mode, 8 types of partition shapes may be present in a CU.For example, in the inter mode, the 2N×2N mode 410, a 2N×N mode 415, anN×2N mode 420, an N×N mode 425, a 2N×nU mode 430, a 2N×nD mode 435, annL×2N mode 440, and an nR×2N mode 445 may be supported.

In an intra mode, the 2N×2N mode 410 and the N×N mode 425 may besupported.

In the 2N×2N mode 410, a PU having a size of 2N×2N may be encoded. ThePU having a size of 2N×2N may mean a PU having a size identical to thatof the CU. For example, the PU having a size of 2N×2N may have a size of64×64, 32×32, 16×16 or 8×8.

In the N×N mode 425, a PU having a size of N×N may be encoded.

For example, in intra prediction, when the size of a PU is 8×8, fourpartitioned PUs may be encoded. The size of each partitioned PU may be4×4.

When a PU is encoded in an intra mode, the PU may be encoded using anyone of multiple intra-prediction modes. For example, HEVC technology mayprovide 35 intra-prediction modes, and the PU may be encoded in any oneof the 35 intra-prediction modes.

Which one of the 2N×2N mode 410 and the N×N mode 425 is to be used toencode the PU may be determined based on rate-distortion cost.

The encoding apparatus 100 may perform an encoding operation on a PUhaving a size of 2N×2N. Here, the encoding operation may be theoperation of encoding the PU in each of multiple intra-prediction modesthat can be used by the encoding apparatus 100. Through the encodingoperation, the optimal intra-prediction mode for a PU having a size of2N×2N may be derived. The optimal intra-prediction mode may be anintra-prediction mode in which a minimum rate-distortion cost occursupon encoding the PU having a size of 2N×2N, among multipleintra-prediction modes that can be used by the encoding apparatus 100.

Further, the encoding apparatus 100 may sequentially perform an encodingoperation on respective PUs obtained from N×N partitioning. Here, theencoding operation may be the operation of encoding a PU in each ofmultiple intra-prediction modes that can be used by the encodingapparatus 100. By means of the encoding operation, the optimalintra-prediction mode for the PU having a size of N×N may be derived.The optimal intra-prediction mode may be an intra-prediction mode inwhich a minimum rate-distortion cost occurs upon encoding the PU havinga size of N×N, among multiple intra-prediction modes that can be used bythe encoding apparatus 100.

The encoding apparatus 100 may determine which of a PU having a size of2N×2N and PUs having sizes of N×N to be encoded based on a comparison ofa rate-distortion cost of the PU having a size of 2N×2N and arate-distortion costs of the PUs having sizes of N×N.

A single CU may be partitioned into one or more PUs, and a PU may bepartitioned into multiple PUs.

For example, when a single PU is partitioned into four PUs, thehorizontal size and vertical size of each of four PUs generated bypartitioning may be half the horizontal size and the vertical size ofthe PU before being partitioned. When a PU having a 32×32 size ispartitioned into four PUs, the size of each of four partitioned PUs maybe 16×16. When a single PU is partitioned into four PUs, it may beconsidered that the PU has been partitioned in a quad-tree structure.

For example, when a single PU is partitioned into two PUs, thehorizontal size or the vertical size of each of two PUs generated bypartitioning may be half the horizontal size or the vertical size of thePU before being partitioned. When a PU having a 32×32 size is verticallypartitioned into two PUs, the size of each of two partitioned PUs may be16×32. When a PU having a 32×32 size is horizontally partitioned intotwo PUs, the size of each of two partitioned PUs may be 32×16. When asingle PU is partitioned into two PUs, it may be considered that the PUhas been partitioned in a binary-tree structure.

FIG. 5 is a diagram illustrating the form of a Transform Unit (TU) thatcan be included in a CU.

A Transform Unit (TU) may have a base unit that is used for a procedure,such as transform, quantization, inverse transform, dequantization,entropy encoding, and entropy decoding, in a CU.

A TU may have a square shape or a rectangular shape. A shape of a TU maybe determined based on a size and/or a shape of a CU.

Among CUs partitioned from the LCU, a CU which is not partitioned intoCUs any further may be partitioned into one or more TUs. Here, thepartition structure of a TU may be a quad-tree structure. For example,as shown in FIG. 5, a single CU 510 may be partitioned one or more timesdepending on the quad-tree structure. By means of this partitioning, thesingle CU 510 may be composed of TUs having various sizes.

It can be considered that when a single CU is split two or more times,the CU is recursively split. Through splitting, a single CU may becomposed of Transform Units (TUs) having various sizes.

Alternatively, a single CU may be split into one or more TUs based onthe number of vertical lines and/or horizontal lines that split the CU.

A CU may be split into symmetric TUs or asymmetric TUs. For splittinginto asymmetric TUs, information about the size and/or shape of each TUmay be signaled from the encoding apparatus 100 to the decodingapparatus 200. Alternatively, the size and/or shape of each TU may bederived from information about the size and/or shape of the CU.

A CU may not be split into TUs. When the CU is not split into TUs, thesize of the CU and the size of a TU may be equal to each other.

A single CU may be partitioned into one or more TUs, and a TU may bepartitioned into multiple TUs.

For example, when a single TU is partitioned into four TUs, thehorizontal size and vertical size of each of four TUs generated bypartitioning may be half the horizontal size and the vertical size ofthe TU before being partitioned. When a TU having a 32×32 size ispartitioned into four TUs, the size of each of four partitioned TUs maybe 16×16. When a single TU is partitioned into four TUs, it may beconsidered that the TU has been partitioned in a quad-tree structure.

For example, when a single TU is partitioned into two TUs, thehorizontal size or the vertical size of each of two TUs generated bypartitioning may be half the horizontal size or the vertical size of theTU before being partitioned. When a TU having a 32×32 size is verticallypartitioned into two TUs, the size of each of two partitioned TUs may be16×32. When a TU having a 32×32 size is horizontally partitioned intotwo TUs, the size of each of two partitioned TUs may be 32×16. When asingle TU is partitioned into two TUs, it may be considered that the TUhas been partitioned in a binary-tree structure.

In a way differing from that illustrated in FIG. 5, a CU may be split.

For example, a single CU may be split into three CUs. The horizontalsizes or vertical sizes of the three CUs generated from splitting may be¼, ½, and ¼, respectively, of the horizontal size or vertical size ofthe original CU before being split.

For example, when a CU having a 32×32 size is vertically split intothree CUs, the sizes of the three CUs generated from the splitting maybe 8×32, 16×32, and 8×32, respectively. In this way, when a single CU issplit into three CUs, it may be considered that the CU is split in theform of a ternary tree.

One of exemplary splitting forms, that is, quad-tree splitting, binarytree splitting, and ternary tree splitting, may be applied to thesplitting of a CU, and multiple splitting schemes may be combined andused together for splitting of a CU. Here, the case where multiplesplitting schemes are combined and used together may be referred to as“complex tree-format splitting”.

FIG. 6 illustrates the splitting of a block according to an example.

In a video encoding and/or decoding process, a target block may besplit, as illustrated in FIG. 6.

For splitting of the target block, an indicator indicating splitinformation may be signaled from the encoding apparatus 100 to thedecoding apparatus 200. The split information may be informationindicating how the target block is split.

The split information may be one or more of a split flag (hereinafterreferred to as “split_flag”), a quad-binary flag (hereinafter referredto as “QB_flag”), a quad-tree flag (hereinafter referred to as“quadtree_flag”), a binary tree flag (hereinafter referred to as“binarytree_flag”), and a binary type flag (hereinafter referred to as“Btype_flag”).

“split_flag” may be a flag indicating whether a block is split. Forexample, a split_flag value of 1 may indicate that the correspondingblock is split. A split_flag value of 0 may indicate that thecorresponding block is not split.

“QB_flag” may be a flag indicating which one of a quad-tree form and abinary tree form corresponds to the shape in which the block is split.For example, a QB_flag value of 0 may indicate that the block is splitin a quad-tree form. A QB_flag value of 1 may indicate that the block issplit in a binary tree form. Alternatively, a QB_flag value of 0 mayindicate that the block is split in a binary tree form. A QB_flag valueof 1 may indicate that the block is split in a quad-tree form.

“quadtree_flag” may be a flag indicating whether a block is split in aquad-tree form. For example, a quadtree_flag value of 1 may indicatethat the block is split in a quad-tree form. A quadtree_flag value of 0may indicate that the block is not split in a quad-tree form.

“binarytree_flag” may be a flag indicating whether a block is split in abinary tree form. For example, a binarytree_flag value of 1 may indicatethat the block is split in a binary tree form. A binarytree_flag valueof 0 may indicate that the block is not split in a binary tree form.

“Btype_flag” may be a flag indicating which one of a vertical split anda horizontal split corresponds to a split direction when a block issplit in a binary tree form. For example, a Btype_flag value of 0 mayindicate that the block is split in a horizontal direction. A Btype_flagvalue of 1 may indicate that a block is split in a vertical direction.Alternatively, a Btype_flag value of 0 may indicate that the block issplit in a vertical direction. A Btype_flag value of 1 may indicate thata block is split in a horizontal direction.

For example, the split information of the block in FIG. 6 may be derivedby signaling at least one of quadtree_flag, binarytree_flag, andBtype_flag, as shown in the following Table 1.

TABLE 1 quadtree_flag binarytree_flag Btype_flag 1 0 1 1 0 0 1 0 1 0 0 00 0 0 0 0 0 0 1 0 1 1 0 0 0 0 0

For example, the split information of the block in FIG. 6 may be derivedby signaling at least one of split_flag, QB_flag and Btype_flag, asshown in the following Table 2.

TABLE 2 split_flag QB_flag Btype_flag 1 0 1 1 1 0 0 1 0 1 1 0 0 0 0 0 01 1 0 1 1 0 0 0 0

The splitting method may be limited only to a quad-tree or to a binarytree depending on the size and/or shape of the block. When thislimitation is applied, split_flag may be a flag indicating whether ablock is split in a quad-tree form or a flag indicating whether a blockis split in a binary tree form. The size and shape of a block may bederived depending on the depth information of the block, and the depthinformation may be signaled from the encoding apparatus 100 to thedecoding apparatus 200.

When the size of a block falls within a specific range, only splittingin a quad-tree form may be possible. For example, the specific range maybe defined by at least one of a maximum block size and a minimum blocksize at which only splitting in a quad-tree form is possible.

Information indicating the maximum block size and the minimum block sizeat which only splitting in a quad-tree form is possible may be signaledfrom the encoding apparatus 100 to the decoding apparatus 200 through abitstream. Further, this information may be signaled for at least one ofunits such as a video, a sequence, a picture, and a slice (or asegment).

Alternatively, the maximum block size and/or the minimum block size maybe fixed sizes predefined by the encoding apparatus 100 and the decodingapparatus 200. For example, when the size of a block is above 64×64 andbelow 256×256, only splitting in a quad-tree form may be possible. Inthis case, split_flag may be a flag indicating whether splitting in aquad-tree form is performed.

When the size of a block falls within the specific range, only splittingin a binary tree form may be possible. For example, the specific rangemay be defined by at least one of a maximum block size and a minimumblock size at which only splitting in a binary tree form is possible.

Information indicating the maximum block size and/or the minimum blocksize at which only splitting in a binary tree form is possible may besignaled from the encoding apparatus 100 to the decoding apparatus 200through a bitstream. Further, this information may be signaled for atleast one of units such as a sequence, a picture, and a slice (or asegment).

Alternatively, the maximum block size and/or the minimum block size maybe fixed sizes predefined by the encoding apparatus 100 and the decodingapparatus 200. For example, when the size of a block is above 8×8 andbelow 16×16, only splitting in a binary tree form may be possible. Inthis case, split_flag may be a flag indicating whether splitting in abinary tree form is performed.

The splitting of a block may be limited by previous splitting. Forexample, when a block is split in a binary tree form and multiplepartition blocks are generated, each partition block may be additionallysplit only in a binary tree form.

When the horizontal size or vertical size of a partition block is a sizethat cannot be split further, the above-described indicator may not besignaled.

FIG. 7 is a diagram for explaining an embodiment of an intra-predictionprocess.

Arrows radially extending from the center of the graph in FIG. 7indicate the prediction directions of intra-prediction modes. Further,numbers appearing near the arrows indicate examples of mode valuesassigned to intra-prediction modes or to the prediction directions ofthe intra-prediction modes.

Intra encoding and/or decoding may be performed using reference samplesof blocks neighboring a target block. The neighboring blocks may beneighboring reconstructed blocks. For example, intra encoding and/ordecoding may be performed using the values of reference samples whichare included in each neighboring reconstructed block or the codingparameters of the neighboring reconstructed block.

The encoding apparatus 100 and/or the decoding apparatus 200 maygenerate a prediction block by performing intra prediction on a targetblock based on information about samples in a target image. When intraprediction is performed, the encoding apparatus 100 and/or the decodingapparatus 200 may generate a prediction block for the target block byperforming intra prediction based on information about samples in thetarget image. When intra prediction is performed, the encoding apparatus100 and/or the decoding apparatus 200 may perform directional predictionand/or non-directional prediction based on at least one reconstructedreference sample.

A prediction block may be a block generated as a result of performingintra prediction. A prediction block may correspond to at least one of aCU, a PU, and a TU.

The unit of a prediction block may have a size corresponding to at leastone of a CU, a PU, and a TU. The prediction block may have a squareshape having a size of 2N×2N or N×N. The size of N×N may include sizesof 4×4, 8×8, 16×16, 32×32, 64×64, or the like.

Alternatively, a prediction block may a square block having a size of2×2, 4×4, 8×8, 16×16, 32×32, 64×64 or the like or a rectangular blockhaving a size of 2×8, 4×8, 2×16, 4×16, 8×16, or the like.

Intra prediction may be performed in consideration of theintra-prediction mode for the target block. The number ofintra-prediction modes that the target block can have may be apredefined fixed value, and may be a value determined differentlydepending on the attributes of a prediction block. For example, theattributes of the prediction block may include the size of theprediction block, the type of prediction block, etc.

For example, the number of intra-prediction modes may be fixed at 35regardless of the size of a prediction block. Alternatively, the numberof intra-prediction modes may be, for example, 3, 5, 9, 17, 34, 35, or36.

The intra-prediction modes may be non-directional modes or directionalmodes. For example, the intra-prediction modes may include twonon-directional modes and 33 directional modes, as shown in FIG. 7.

The two non-directional modes may include a DC mode and a planar mode.

The directional modes may be prediction modes having a specificdirection or a specific angle.

The intra-prediction modes may each be represented by at least one of amode number, a mode value, and a mode angle. The number ofintra-prediction modes may be M. The value of M may be 1 or more. Inother words, the number of intra-prediction modes may be M, whichincludes the number of non-directional modes and the number ofdirectional modes.

The number of intra-prediction modes may be fixed to M regardless of thesize and/or the color component of a block. For example, the number ofintra-prediction modes may be fixed at any one of 35 and 67 regardlessof the size of a block.

Alternatively, the number of intra-prediction modes may differ dependingon the size of a block and/or the type of color component.

For example, the larger the size of the block, the greater the number ofintra-prediction modes. Alternatively, the larger the size of the block,the smaller the number of intra-prediction modes. When the size of theblock is 4×4 or 8×8, the number of intra-prediction modes may be 67.When the size of the block is 16×16, the number of intra-predictionmodes may be 35. When the size of the block is 32×32, the number ofintra-prediction modes may be 19. When the size of a block is 64×64, thenumber of intra-prediction modes may be 7.

For example, the number of intra prediction modes may differ dependingon whether a color component is a luma signal or a chroma signal.Alternatively, the number of intra-prediction modes corresponding to aluma component block may be greater than the number of intra-predictionmodes corresponding to a chroma component block.

For example, in a vertical mode having a mode value of 26, predictionmay be performed in a vertical direction based on the pixel value of areference sample. For example, in a horizontal mode having a mode valueof 10, prediction may be performed in a horizontal direction based onthe pixel value of a reference sample.

Even in directional modes other than the above-described mode, theencoding apparatus 100 and the decoding apparatus 200 may perform intraprediction on a target unit using reference samples depending on anglescorresponding to the directional modes.

Intra-prediction modes located on a right side with respect to thevertical mode may be referred to as ‘vertical-right modes’.Intra-prediction modes located below the horizontal mode may be referredto as ‘horizontal-below modes’. For example, in FIG. 7, theintra-prediction modes in which a mode value is one of 27, 28, 29, 30,31, 32, 33, and 34 may be vertical-right modes 613. Intra-predictionmodes in which a mode value is one of 2, 3, 4, 5, 6, 7, 8, and 9 may behorizontal-below modes 616.

The non-directional mode may include a DC mode and a planar mode. Forexample, a value of the DC mode may be 1. A value of the planar mode maybe 0.

The directional mode may include an angular mode. Among the plurality ofthe intra prediction modes, remaining modes except for the DC mode andthe planar mode may be directional modes.

When the intra-prediction mode is a DC mode, a prediction block may begenerated based on the average of pixel values of a plurality ofreference pixels. For example, a value of a pixel of a prediction blockmay be determined based on the average of pixel values of a plurality ofreference pixels.

The number of above-described intra-prediction modes and the mode valuesof respective intra-prediction modes are merely exemplary. The number ofabove-described intra-prediction modes and the mode values of respectiveintra-prediction modes may be defined differently depending on theembodiments, implementation and/or requirements.

In order to perform intra prediction on a target block, the step ofchecking whether samples included in a reconstructed neighboring blockcan be used as reference samples of a target block may be performed.When a sample that cannot be used as a reference sample of the targetblock is present among samples in the neighboring block, a valuegenerated via copying and/or interpolation that uses at least one samplevalue, among the samples included in the reconstructed neighboringblock, may replace the sample value of the sample that cannot be used asthe reference sample. When the value generated via copying and/orinterpolation replaces the sample value of the existing sample, thesample may be used as the reference sample of the target block.

In intra prediction, a filter may be applied to at least one of areference sample and a prediction sample based on at least one of theintra-prediction mode and the size of the target block.

The type of filter to be applied to at least one of a reference sampleand a prediction sample may differ depending on at least one of theintra-prediction mode of a target block, the size of the target block,and the shape of the target block. The types of filters may beclassified depending on one or more of the number of filter taps, thevalue of a filter coefficient, and filter strength.

When the intra-prediction mode is a planar mode, a sample value of aprediction target block may be generated using a weighted sum of anabove reference sample of the target block, a left reference sample ofthe target block, an above-right reference sample of the target block,and a below-left reference sample of the target block depending on thelocation of the prediction target sample in the prediction block whenthe prediction block of the target block is generated.

When the intra-prediction mode is a DC mode, the average of referencesamples above the target block and the reference samples to the left ofthe target block may be used when the prediction block of the targetblock is generated. Also, filtering using the values of referencesamples may be performed on specific rows or specific columns in thetarget block. The specific rows may be one or more upper rows adjacentto the reference sample. The specific columns may be one or more leftcolumns adjacent to the reference sample.

When the intra-prediction mode is a directional mode, a prediction blockmay be generated using the above reference samples, left referencesamples, above-right reference sample and/or below-left reference sampleof the target block.

In order to generate the above-described prediction sample,real-number-based interpolation may be performed.

The intra-prediction mode of the target block may be predicted fromintra prediction mode of a neighboring block adjacent to the targetblock, and the information used for prediction may beentropy-encoded/decoded.

For example, when the intra-prediction modes of the target block and theneighboring block are identical to each other, it may be signaled, usinga predefined flag, that the intra-prediction modes of the target blockand the neighboring block are identical.

For example, an indicator for indicating an intra-prediction modeidentical to that of the target block, among intra-prediction modes ofmultiple neighboring blocks, may be signaled.

When the intra-prediction modes of the target block and a neighboringblock are different from each other, information about theintra-prediction mode of the target block may be encoded and/or decodedusing entropy encoding and/or decoding.

FIG. 8 is a diagram for explaining the locations of reference samplesused in an intra-prediction procedure.

FIG. 8 illustrates the locations of reference samples used for intraprediction of a target block. Referring to FIG. 8, reconstructedreference samples used for intra prediction of the target block mayinclude below-left reference samples 831, left reference samples 833, anabove-left corner reference sample 835, above reference samples 837, andabove-right reference samples 839.

For example, the left reference samples 833 may mean reconstructedreference pixels adjacent to the left side of the target block. Theabove reference samples 837 may mean reconstructed reference pixelsadjacent to the top of the target block. The above-left corner referencesample 835 may mean a reconstructed reference pixel located at theabove-left corner of the target block. The below-left reference samples831 may mean reference samples located below a left sample line composedof the left reference samples 833, among samples located on the sameline as the left sample line. The above-right reference samples 839 maymean reference samples located to the right of an above sample linecomposed of the above reference samples 837, among samples located onthe same line as the above sample line.

When the size of a target block is N×N, the numbers of the below-leftreference samples 831, the left reference samples 833, the abovereference samples 837, and the above-right reference samples 839 mayeach be N.

By performing intra prediction on the target block, a prediction blockmay be generated. The generation of the prediction block may include thedetermination of the values of pixels in the prediction block. The sizesof the target block and the prediction block may be equal.

The reference samples used for intra prediction of the target block mayvary depending on the intra-prediction mode of the target block. Thedirection of the intra-prediction mode may represent a dependencerelationship between the reference samples and the pixels of theprediction block. For example, the value of a specified reference samplemay be used as the values of one or more specified pixels in theprediction block. In this case, the specified reference sample and theone or more specified pixels in the prediction block may be the sampleand pixels which are positioned in a straight line in the direction ofan intra-prediction mode. In other words, the value of the specifiedreference sample may be copied as the value of a pixel located in adirection reverse to the direction of the intra-prediction mode.Alternatively, the value of a pixel in the prediction block may be thevalue of a reference sample located in the direction of theintra-prediction mode with respect to the location of the pixel.

In an example, when the intra-prediction mode of a target block is avertical mode having a mode value of 26, the above reference samples 837may be used for intra prediction. When the intra-prediction mode is thevertical mode, the value of a pixel in the prediction block may be thevalue of a reference sample vertically located above the location of thepixel. Therefore, the above reference samples 837 adjacent to the top ofthe target block may be used for intra prediction. Furthermore, thevalues of pixels in one row of the prediction block may be identical tothose of the above reference samples 837.

In an example, when the intra-prediction mode of a target block is ahorizontal mode having a mode value of 10, the left reference samples833 may be used for intra prediction. When the intra-prediction mode isthe horizontal mode, the value of a pixel in the prediction block may bethe value of a reference sample horizontally located left to thelocation of the pixel. Therefore, the left reference samples 833adjacent to the left of the target block may be used for intraprediction. Furthermore, the values of pixels in one column of theprediction block may be identical to those of the left reference samples833.

In an example, when the mode value of the intra-prediction mode of thecurrent block is 18, at least some of the left reference samples 833,the above-left corner reference sample 835, and at least some of theabove reference samples 837 may be used for intra prediction. When themode value of the intra-prediction mode is 18, the value of a pixel inthe prediction block may be the value of a reference sample diagonallylocated at the above-left corner of the pixel.

Further, At least a part of the above-right reference samples 839 may beused for intra prediction in a case that a intra prediction mode havinga mode value of 27, 28, 29, 30, 31, 32, 33 or 34 is used.

Further, At least a part of the below-left reference samples 831 may beused for intra prediction in a case that a intra prediction mode havinga mode value of 2, 3, 4, 5, 6, 7, 8 or 9 is used.

Further, the above-left corner reference sample 835 may be used forintra prediction in a case that a intra prediction mode of which a modevalue is a value ranging from 11 to 25.

The number of reference samples used to determine the pixel value of onepixel in the prediction block may be either 1, or 2 or more.

As described above, the pixel value of a pixel in the prediction blockmay be determined depending on the location of the pixel and thelocation of a reference sample indicated by the direction of theintra-prediction mode. When the location of the pixel and the locationof the reference sample indicated by the direction of theintra-prediction mode are integer positions, the value of one referencesample indicated by an integer position may be used to determine thepixel value of the pixel in the prediction block.

When the location of the pixel and the location of the reference sampleindicated by the direction of the intra-prediction mode are not integerpositions, an interpolated reference sample based on two referencesamples closest to the location of the reference sample may begenerated. The value of the interpolated reference sample may be used todetermine the pixel value of the pixel in the prediction block. In otherwords, when the location of the pixel in the prediction block and thelocation of the reference sample indicated by the direction of theintra-prediction mode indicate the location between two referencesamples, an interpolated value based on the values of the two samplesmay be generated.

The prediction block generated via prediction may not be identical to anoriginal target block. In other words, there may be a prediction errorwhich is the difference between the target block and the predictionblock, and there may also be a prediction error between the pixel of thetarget block and the pixel of the prediction block.

Hereinafter, the terms “difference”, “error”, and “residual” may be usedto have the same meaning, and may be used interchangeably with eachother.

For example, in the case of directional intra prediction, the longer thedistance between the pixel of the prediction block and the referencesample, the greater the prediction error that may occur. Such aprediction error may result in discontinuity between the generatedprediction block and neighboring blocks.

In order to reduce the prediction error, filtering for the predictionblock may be used. Filtering may be configured to adaptively apply afilter to an area, regarded as having a large prediction error, in theprediction block. For example, the area regarded as having a largeprediction error may be the boundary of the prediction block. Further,an area regarded as having a large prediction error in the predictionblock may differ depending on the intra-prediction mode, and thecharacteristics of filters may also differ depending thereon.

FIG. 9 is a diagram for explaining an embodiment of an inter predictionprocedure.

The rectangles shown in FIG. 9 may represent images (or pictures).Further, in FIG. 9, arrows may represent prediction directions. That is,each image may be encoded and/or decoded depending on the predictiondirection.

Images may be classified into an Intra Picture (I picture), aUni-prediction Picture or Predictive Coded Picture (P picture), and aBi-prediction Picture or Bi-predictive Coded Picture (B picture)depending on the encoding type. Each picture may be encoded and/ordecoded depending on the encoding type thereof.

When a target image that is the target to be encoded is an I picture,the target image may be encoded using data contained in the image itselfwithout inter prediction that refers to other images. For example, an Ipicture may be encoded only via intra prediction.

When a target image is a P picture, the target image may be encoded viainter prediction, which uses reference pictures existing in onedirection. Here, the one direction may be a forward direction or abackward direction.

When a target image is a B picture, the image may be encoded via interprediction that uses reference pictures existing in two directions, ormay be encoded via inter prediction that uses reference picturesexisting in one of a forward direction and a backward direction. Here,the two directions may be the forward direction and the backwarddirection.

A P picture and a B picture that are encoded and/or decoded usingreference pictures may be regarded as images in which inter predictionis used.

Below, inter prediction in an inter mode according to an embodiment willbe described in detail.

Inter prediction may be performed using motion information.

In an inter mode, the encoding apparatus 100 may perform interprediction and/or motion compensation on a target block. The decodingapparatus 200 may perform inter prediction and/or motion compensation,corresponding to inter prediction and/or motion compensation performedby the encoding apparatus 100, on a target block.

Motion information of the target block may be individually derived bythe encoding apparatus 100 and the decoding apparatus 200 during theinter prediction. The motion information may be derived using motioninformation of a reconstructed neighboring block, motion information ofa col block, and/or motion information of a block adjacent to the colblock.

For example, the encoding apparatus 100 or the decoding apparatus 200may perform prediction and/or motion compensation by using motioninformation of a spatial candidate and/or a temporal candidate as motioninformation of the target block. The target block may mean a PU and/or aPU partition.

A spatial candidate may be a reconstructed block which is spatiallyadjacent to the target block.

A temporal candidate may be a reconstructed block corresponding to thetarget block in a previously reconstructed co-located picture (colpicture).

In inter prediction, the encoding apparatus 100 and the decodingapparatus 200 may improve encoding efficiency and decoding efficiency byutilizing the motion information of a spatial candidate and/or atemporal candidate. The motion information of a spatial candidate may bereferred to as ‘spatial motion information’. The motion information of atemporal candidate may be referred to as ‘temporal motion information’.

Below, the motion information of a spatial candidate may be the motioninformation of a PU including the spatial candidate. The motioninformation of a temporal candidate may be the motion information of aPU including the temporal candidate. The motion information of acandidate block may be the motion information of a PU including thecandidate block.

Inter prediction may be performed using a reference picture.

The reference picture may be at least one of a picture previous to atarget picture and a picture subsequent to the target picture. Thereference picture may be an image used for the prediction of the targetblock.

In inter prediction, a region in the reference picture may be specifiedby utilizing a reference picture index (or refIdx) for indicating areference picture, a motion vector, which will be described later, etc.Here, the region specified in the reference picture may indicate areference block.

Inter prediction may select a reference picture, and may also select areference block corresponding to the target block from the referencepicture. Further, inter prediction may generate a prediction block forthe target block using the selected reference block.

The motion information may be derived during inter prediction by each ofthe encoding apparatus 100 and the decoding apparatus 200.

A spatial candidate may be a block 1) which is present in a targetpicture, 2) which has been previously reconstructed via encoding and/ordecoding, and 3) which is adjacent to the target block or is located atthe corner of the target block. Here, the “block located at the cornerof the target block” may be either a block vertically adjacent to aneighboring block that is horizontally adjacent to the target block, ora block horizontally adjacent to a neighboring block that is verticallyadjacent to the target block. Further, “block located at the corner ofthe target block” may have the same meaning as “block adjacent to thecorner of the target block”. The meaning of “block located at the cornerof the target block” may be included in the meaning of “block adjacentto the target block”.

For example, a spatial candidate may be a reconstructed block located tothe left of the target block, a reconstructed block located above thetarget block, a reconstructed block located at the below-left corner ofthe target block, a reconstructed block located at the above-rightcorner of the target block, or a reconstructed block located at theabove-left corner of the target block.

Each of the encoding apparatus 100 and the decoding apparatus 200 mayidentify a block present at the location spatially corresponding to thetarget block in a col picture. The location of the target block in thetarget picture and the location of the identified block in the colpicture may correspond to each other.

Each of the encoding apparatus 100 and the decoding apparatus 200 maydetermine a col block present at the predefined relative location forthe identified block to be a temporal candidate. The predefined relativelocation may be a location present inside and/or outside the identifiedblock.

For example, the col block may include a first col block and a secondcol block. When the coordinates of the identified block are (xP, yP) andthe size of the identified block is represented by (nPSW, nPSH), thefirst col block may be a block located at coordinates (xP+nPSW,yP+nPSH). The second col block may be a block located at coordinates(xP+(nPSW>>1), yP+(nPSH>>1)). The second col block may be selectivelyused when the first col block is unavailable.

The motion vector of the target block may be determined based on themotion vector of the col block. Each of the encoding apparatus 100 andthe decoding apparatus 200 may scale the motion vector of the col block.The scaled motion vector of the col block may be used as the motionvector of the target block. Further, a motion vector for the motioninformation of a temporal candidate stored in a list may be a scaledmotion vector.

The ratio of the motion vector of the target block to the motion vectorof the col block may be identical to the ratio of a first distance to asecond distance. The first distance may be the distance between thereference picture and the target picture of the target block. The seconddistance may be the distance between the reference picture and the colpicture of the col block.

The scheme for deriving motion information may change depending on theinter-prediction mode of a target block. For example, asinter-prediction modes applied for inter prediction, an Advanced MotionVector Predictor (AMVP) mode, a merge mode, a skip mode, a currentpicture reference mode, etc. may be present. The merge mode may also bereferred to as a “motion merge mode”. Individual modes will be describedin detail below.

1) AMVP Mode

When an AMVP mode is used, the encoding apparatus 100 may search aneighboring region of a target block for a similar block. The encodingapparatus 100 may acquire a prediction block by performing prediction onthe target block using motion information of the found similar block.The encoding apparatus 100 may encode a residual block, which is thedifference between the target block and the prediction block.

1-1) Creation of List of Prediction Motion Vector Candidates

When an AMVP mode is used as the prediction mode, each of the encodingapparatus 100 and the decoding apparatus 200 may create a list ofprediction motion vector candidates using the motion vector of a spatialcandidate, the motion vector of a temporal candidate, and a zero vector.The prediction motion vector candidate list may include one or moreprediction motion vector candidates. At least one of the motion vectorof a spatial candidate, the motion vector of a temporal candidate, and azero vector may be determined and used as a prediction motion vectorcandidate.

Hereinafter, the terms “prediction motion vector (candidate)” and“motion vector (candidate)” may be used to have the same meaning, andmay be used interchangeably with each other.

Hereinafter, the terms “prediction motion vector candidate” and “AMVPcandidate” may be used to have the same meaning, and may be usedinterchangeably with each other.

Hereinafter, the terms “prediction motion vector candidate list” and“AMVP candidate list” may be used to have the same meaning, and may beused interchangeably with each other.

Spatial candidates may include a reconstructed spatial neighboringblock. In other words, the motion vector of the reconstructedneighboring block may be referred to as a “spatial prediction motionvector candidate”.

Temporal candidates may include a col block and a block adjacent to thecol block. In other words, the motion vector of the col block or themotion vector of the block adjacent to the col block may be referred toas a “temporal prediction motion vector candidate”.

The zero vector may be a (0, 0) motion vector.

The prediction motion vector candidates may be motion vector predictorsfor predicting a motion vector. Also, in the encoding apparatus 100,each prediction motion vector candidate may be an initial searchlocation for a motion vector.

1-2) Search for Motion Vectors that Use List of Prediction Motion VectorCandidates

The encoding apparatus 100 may determine the motion vector to be used toencode a target block within a search range using a list of predictionmotion vector candidates. Further, the encoding apparatus 100 maydetermine a prediction motion vector candidate to be used as theprediction motion vector of the target block, among prediction motionvector candidates present in the prediction motion vector candidatelist.

The motion vector to be used to encode the target block may be a motionvector that can be encoded at minimum cost.

Further, the encoding apparatus 100 may determine whether to use theAMVP mode to encode the target block.

1-3) Transmission of Inter-Prediction Information

The encoding apparatus 100 may generate a bitstream includinginter-prediction information required for inter prediction. The decodingapparatus 200 may perform inter prediction on the target block using theinter-prediction information of the bitstream.

The inter-prediction information may contain 1) mode informationindicating whether an AMVP mode is used, 2) a prediction motion vectorindex, 3) a Motion Vector Difference (MVD), 4) a reference direction,and 5) a reference picture index.

Hereinafter, the terms “prediction motion vector index” and “AMVP index”may be used to have the same meaning, and may be used interchangeablywith each other.

Further, the inter-prediction information may contain a residual signal.

The decoding apparatus 200 may acquire a prediction motion vector index,an MVD, a reference direction, and a reference picture index from thebitstream through entropy decoding when mode information indicates thatthe AMVP mode is used.

The prediction motion vector index may indicate a prediction motionvector candidate to be used for the prediction of a target block, amongprediction motion vector candidates included in the prediction motionvector candidate list.

1-4) Inter Prediction in AMVP Mode that Uses Inter-PredictionInformation

The decoding apparatus 200 may derive prediction motion vectorcandidates using a prediction motion vector candidate list, and maydetermine the motion information of a target block based on the derivedprediction motion vector candidates.

The decoding apparatus 200 may determine a motion vector candidate forthe target block, among the prediction motion vector candidates includedin the prediction motion vector candidate list, using a predictionmotion vector index. The decoding apparatus 200 may select a predictionmotion vector candidate, indicated by the prediction motion vectorindex, from among prediction motion vector candidates included in theprediction motion vector candidate list, as the prediction motion vectorof the target block.

The motion vector to be actually used for inter prediction of the targetblock may not match the prediction motion vector. In order to indicatethe difference between the motion vector to be actually used for interprediction of the target block and the prediction motion vector, an MVDmay be used. The encoding apparatus 100 may derive a prediction motionvector similar to the motion vector to be actually used for interprediction of the target block so as to use an MVD that is as small aspossible.

An MVD may be the difference between the motion vector of the targetblock and the prediction motion vector. The encoding apparatus 100 maycalculate an MVD and may entropy-encode the MVD.

The MVD may be transmitted from the encoding apparatus 100 to thedecoding apparatus 200 through a bitstream. The decoding apparatus 200may decode the received MVD. The decoding apparatus 200 may derive themotion vector of the target block by summing the decoded MVD and theprediction motion vector. In other words, the motion vector of thetarget block derived by the decoding apparatus 200 may be the sum of theentropy-decoded MVD and the motion vector candidate.

The reference direction may indicate a list of reference pictures to beused for prediction of the target block. For example, the referencedirection may indicate one of a reference picture list L0 and areference picture list L1.

The reference direction merely indicates the reference picture list tobe used for prediction of the target block, and may not mean that thedirections of reference pictures are limited to a forward direction or abackward direction. In other words, each of the reference picture listL0 and the reference picture list L1 may include pictures in a forwarddirection and/or a backward direction.

That the reference direction is unidirectional may mean that a singlereference picture list is used. That the reference direction isbidirectional may mean that two reference picture lists are used. Inother words, the reference direction may indicate one of the case whereonly the reference picture list L0 is used, the case where only thereference picture list L1 is used, and the case where two referencepicture lists are used.

The reference picture index may indicate a reference picture to be usedfor prediction of a target block, among reference pictures in thereference picture list. The reference picture index may beentropy-encoded by the encoding apparatus 100. The entropy-encodedreference picture index may be signaled to the decoding apparatus 200 bythe encoding apparatus 100 through a bitstream.

When two reference picture lists are used to predict the target block, asingle reference picture index and a single motion vector may be usedfor each of the reference picture lists. Further, when two referencepicture lists are used to predict the target block, two predictionblocks may be specified for the target block. For example, the (final)prediction block of the target block may be generated using the averageor weighted sum of the two prediction blocks for the target block.

The motion vector of the target block may be derived by the predictionmotion vector index, the MVD, the reference direction, and the referencepicture index.

The decoding apparatus 200 may generate a prediction block for thetarget block based on the derived motion vector and the referencepicture index. For example, the prediction block may be a referenceblock, indicated by the derived motion vector, in the reference pictureindicated by the reference picture index.

Since the prediction motion vector index and the MVD are encoded withoutthe motion vector itself of the target block being encoded, the numberof bits transmitted from the encoding apparatus 100 to the decodingapparatus 200 may be decreased, and encoding efficiency may be improved.

For the target block, the motion information of reconstructedneighboring blocks may be used. In a specific inter-prediction mode, theencoding apparatus 100 may not separately encode the actual motioninformation of the target block. The motion information of the targetblock is not encoded, and additional information that enables the motioninformation of the target block to be derived using the motioninformation of reconstructed neighboring blocks may be encoded instead.As the additional information is encoded, the number of bits transmittedto the decoding apparatus 200 may be decreased, and encoding efficiencymay be improved.

For example, as inter-prediction modes in which the motion informationof the target block is not directly encoded, there may be a skip modeand/or a merge mode. Here, each of the encoding apparatus 100 and thedecoding apparatus 200 may use an identifier and/or an index thatindicates a unit, the motion information of which is to be used as themotion information of the target unit, among reconstructed neighboringunits.

2) Merge Mode

As a scheme for deriving the motion information of a target block, thereis merging. The term “merging” may mean the merging of the motion ofmultiple blocks. “Merging” may mean that the motion information of oneblock is also applied to other blocks. In other words, a merge mode maybe a mode in which the motion information of the target block is derivedfrom the motion information of a neighboring block.

When a merge mode is used, the encoding apparatus 100 may predict themotion information of a target block using the motion information of aspatial candidate and/or the motion information of a temporal candidate.The spatial candidate may include a reconstructed spatial neighboringblock that is spatially adjacent to the target block. The spatialneighboring block may include a left adjacent block and an aboveadjacent block. The temporal candidate may include a col block. Theterms “spatial candidate” and “spatial merge candidate” may be used tohave the same meaning, and may be used interchangeably with each other.The terms “temporal candidate” and “temporal merge candidate” may beused to have the same meaning, and may be used interchangeably with eachother.

The encoding apparatus 100 may acquire a prediction block viaprediction. The encoding apparatus 100 may encode a residual block,which is the difference between the target block and the predictionblock.

2-1) Creation of Merge Candidate List

When the merge mode is used, each of the encoding apparatus 100 and thedecoding apparatus 200 may create a merge candidate list using themotion information of a spatial candidate and/or the motion informationof a temporal candidate. The motion information may include 1) a motionvector, 2) a reference picture index, and 3) a reference direction. Thereference direction may be unidirectional or bidirectional.

The merge candidate list may include merge candidates. The mergecandidates may be motion information. In other words, the mergecandidate list may be a list in which pieces of motion information arestored.

The merge candidates may be pieces of motion information of temporalcandidates and/or spatial candidates. Further, the merge candidate listmay include new merge candidates generated by a combination of mergecandidates that are already present in the merge candidate list. Inother words, the merge candidate list may include new motion informationgenerated by a combination of pieces of motion information previouslypresent in the merge candidate list.

The merge candidates may be specific modes deriving inter predictioninformation. The merge candidate may be information indicating aspecific mode deriving inter prediction information. Inter predictioninformation of a target block may be derived according to a specificmode which the merge candidate indicates. Furthermore, the specific modemay include a process of deriving a series of inter predictioninformation. This specific mode may be an inter prediction informationderivation mode or a motion information derivation mode.

The inter prediction information of the target block may be derivedaccording to the mode indicated by the merge candidate selected by themerge index among the merge candidates in the merge candidate list

For example, the motion information derivation modes in the mergecandidate list may be at least one of 1) motion information derivationmode for a sub-block unit and 2) an affine motion information derivationmode.

Furthermore, the merge candidate list may include motion information ofa zero vector. The zero vector may also be referred to as a “zero-mergecandidate”.

In other words, pieces of motion information in the merge candidate listmay be at least one of 1) motion information of a spatial candidate, 2)motion information of a temporal candidate, 3) motion informationgenerated by a combination of pieces of motion information previouslypresent in the merge candidate list, and 4) a zero vector.

Motion information may include 1) a motion vector, 2) a referencepicture index, and 3) a reference direction. The reference direction mayalso be referred to as an “inter-prediction indicator”. The referencedirection may be unidirectional or bidirectional. The unidirectionalreference direction may indicate L0 prediction or L1 prediction.

The merge candidate list may be created before prediction in the mergemode is performed.

The number of merge candidates in the merge candidate list may bepredefined. Each of the encoding apparatus 100 and the decodingapparatus 200 may add merge candidates to the merge candidate listdepending on the predefined scheme and predefined priorities so that themerge candidate list has a predefined number of merge candidates. Themerge candidate list of the encoding apparatus 100 and the mergecandidate list of the decoding apparatus 200 may be made identical toeach other using the predefined scheme and the predefined priorities.

Merging may be applied on a CU basis or a PU basis. When merging isperformed on a CU basis or a PU basis, the encoding apparatus 100 maytransmit a bitstream including predefined information to the decodingapparatus 200. For example, the predefined information may contain 1)information indicating whether to perform merging for individual blockpartitions, and 2) information about a block with which merging is to beperformed, among blocks that are spatial candidates and/or temporalcandidates for the target block.

2-2) Search for Motion Vector that Uses Merge Candidate List

The encoding apparatus 100 may determine merge candidates to be used toencode a target block. For example, the encoding apparatus 100 mayperform prediction on the target block using merge candidates in themerge candidate list, and may generate residual blocks for the mergecandidates. The encoding apparatus 100 may use a merge candidate thatincurs the minimum cost in prediction and in the encoding of residualblocks to encode the target block.

Further, the encoding apparatus 100 may determine whether to use a mergemode to encode the target block.

2-3) Transmission of Inter-Prediction Information

The encoding apparatus 100 may generate a bitstream that includesinter-prediction information required for inter prediction. The encodingapparatus 100 may generate entropy-encoded inter-prediction informationby performing entropy encoding on inter-prediction information, and maytransmit a bitstream including the entropy-encoded inter-predictioninformation to the decoding apparatus 200. Through the bitstream, theentropy-encoded inter-prediction information may be signaled to thedecoding apparatus 200 by the encoding apparatus 100.

The decoding apparatus 200 may perform inter prediction on the targetblock using the inter-prediction information of the bitstream.

The inter-prediction information may contain 1) mode informationindicating whether a merge mode is used and 2) a merge index.

Further, the inter-prediction information may contain a residual signal.

The decoding apparatus 200 may acquire the merge index from thebitstream only when the mode information indicates that the merge modeis used.

The mode information may be a merge flag. The unit of the modeinformation may be a block. Information about the block may include modeinformation, and the mode information may indicate whether a merge modeis applied to the block.

The merge index may indicate a merge candidate to be used for theprediction of the target block, among merge candidates included in themerge candidate list. Alternatively, the merge index may indicate ablock with which the target block is to be merged, among neighboringblocks spatially or temporally adjacent to the target block.

The encoding apparatus 100 may select a merge candidate having thehighest encoding performance among the merge candidates included in themerge candidate list and set a value of the merge index to indicate theselected merge candidate.

2-4) Inter Prediction of Merge Mode that Uses Inter-PredictionInformation

The decoding apparatus 200 may perform prediction on the target blockusing the merge candidate indicated by the merge index, among mergecandidates included in the merge candidate list.

The motion vector of the target block may be specified by the motionvector, reference picture index, and reference direction of the mergecandidate indicated by the merge index.

3) Skip Mode

A skip mode may be a mode in which the motion information of a spatialcandidate or the motion information of a temporal candidate is appliedto the target block without change. Also, the skip mode may be a mode inwhich a residual signal is not used. In other words, when the skip modeis used, a reconstructed block may be a prediction block.

The difference between the merge mode and the skip mode lies in whetheror not a residual signal is transmitted or used. That is, the skip modemay be similar to the merge mode except that a residual signal is nottransmitted or used.

When the skip mode is used, the encoding apparatus 100 may transmitinformation about a block, the motion information of which is to be usedas the motion information of the target block, among blocks that arespatial candidates or temporal candidates, to the decoding apparatus 200through a bitstream. The encoding apparatus 100 may generateentropy-encoded information by performing entropy encoding on theinformation, and may signal the entropy-encoded information to thedecoding apparatus 200 through a bitstream.

Further, when the skip mode is used, the encoding apparatus 100 may nottransmit other syntax information, such as an MVD, to the decodingapparatus 200. For example, when the skip mode is used, the encodingapparatus 100 may not signal a syntax element related to at least one ofan MVC, a coded block flag, and a transform coefficient level to thedecoding apparatus 200.

3-1) Creation of Merge Candidate List

The skip mode may also use a merge candidate list. In other words, amerge candidate list may be used both in the merge mode and in the skipmode. In this aspect, the merge candidate list may also be referred toas a “skip candidate list” or a “merge/skip candidate list”.

Alternatively, the skip mode may use an additional candidate listdifferent from that of the merge mode. In this case, in the followingdescription, a merge candidate list and a merge candidate may bereplaced with a skip candidate list and a skip candidate, respectively.

The merge candidate list may be created before prediction in the skipmode is performed.

3-2) Search for Motion Vector that Uses Merge Candidate List

The encoding apparatus 100 may determine the merge candidates to be usedto encode a target block. For example, the encoding apparatus 100 mayperform prediction on the target block using the merge candidates in amerge candidate list. The encoding apparatus 100 may use a mergecandidate that incurs the minimum cost in prediction to encode thetarget block.

Further, the encoding apparatus 100 may determine whether to use a skipmode to encode the target block.

3-3) Transmission of Inter-Prediction Information

The encoding apparatus 100 may generate a bitstream that includesinter-prediction information required for inter prediction. The decodingapparatus 200 may perform inter prediction on the target block using theinter-prediction information of the bitstream.

The inter-prediction information may include 1) mode informationindicating whether a skip mode is used, and 2) a skip index.

The skip index may be identical to the above-described merge index.

When the skip mode is used, the target block may be encoded withoutusing a residual signal. The inter-prediction information may notcontain a residual signal. Alternatively, the bitstream may not includea residual signal.

The decoding apparatus 200 may acquire a skip index from the bitstreamonly when the mode information indicates that the skip mode is used. Asdescribed above, a merge index and a skip index may be identical to eachother. The decoding apparatus 200 may acquire the skip index from thebitstream only when the mode information indicates that the merge modeor the skip mode is used.

The skip index may indicate the merge candidate to be used for theprediction of the target block, among the merge candidates included inthe merge candidate list.

3-4) Inter Prediction in Skip Mode that Uses Inter-PredictionInformation

The decoding apparatus 200 may perform prediction on the target blockusing a merge candidate indicated by a skip index, among the mergecandidates included in a merge candidate list.

The motion vector of the target block may be specified by the motionvector, reference picture index, and reference direction of the mergecandidate indicated by the skip index.

4) Current Picture Reference Mode

The current picture reference mode may denote a prediction mode thatuses a previously reconstructed region in a target picture to which atarget block belongs.

A motion vector for specifying the previously reconstructed region maybe used. Whether the target block has been encoded in the currentpicture reference mode may be determined using the reference pictureindex of the target block.

A flag or index indicating whether the target block is a block encodedin the current picture reference mode may be signaled by the encodingapparatus 100 to the decoding apparatus 200. Alternatively, whether thetarget block is a block encoded in the current picture reference modemay be inferred through the reference picture index of the target block.

When the target block is encoded in the current picture reference mode,the target picture may exist at a fixed location or an arbitrarylocation in a reference picture list for the target block.

For example, the fixed location may be either a location where a valueof the reference picture index is 0 or the last location.

When the target picture exists at an arbitrary location in the referencepicture list, an additional reference picture index indicating such anarbitrary location may be signaled by the encoding apparatus 100 to thedecoding apparatus 200.

In the above-described AMVP mode, merge mode, and skip mode, motioninformation to be used for the prediction of a target block may bespecified, among pieces of motion information in the list, using theindex of the list.

In order to improve encoding efficiency, the encoding apparatus 100 maysignal only the index of an element that incurs the minimum cost ininter prediction of the target block, among elements in the list. Theencoding apparatus 100 may encode the index, and may signal the encodedindex.

Therefore, the above-described lists (i.e. the prediction motion vectorcandidate list and the merge candidate list) must be able to be derivedby the encoding apparatus 100 and the decoding apparatus 200 using thesame scheme based on the same data. Here, the same data may include areconstructed picture and a reconstructed block. Further, in order tospecify an element using an index, the order of the elements in the listmust be fixed.

FIG. 10 illustrates spatial candidates according to an embodiment.

In FIG. 10, the locations of spatial candidates are illustrated.

The large block in the center of the drawing may denote a target block.Five small blocks may denote spatial candidates.

The coordinates of the target block may be (xP, yP), and the size of thetarget block may be represented by (nPSW, nPSH).

Spatial candidate A₀ may be a block adjacent to the below-left corner ofthe target block. A₀ may be a block that occupies pixels located atcoordinates (xP−1, yP+nPSH+1).

Spatial candidate A₁ may be a block adjacent to the left of the targetblock. A₁ may be a lowermost block, among blocks adjacent to the left ofthe target block. Alternatively, A₁ may be a block adjacent to the topof A₀. A₁ may be a block that occupies pixels located at coordinates(xP−1, yP+nPSH).

Spatial candidate B₀ may be a block adjacent to the above-right cornerof the target block. B₀ may be a block that occupies pixels located atcoordinates (xP+nPSW+1, yP−1).

Spatial candidate B₁ may be a block adjacent to the top of the targetblock. B₁ may be a rightmost block, among blocks adjacent to the top ofthe target block. Alternatively, B₁ may be a block adjacent to the leftof B₀. B₁ may be a block that occupies pixels located at coordinates(xP+nPSW, yP−1).

Spatial candidate B₂ may be a block adjacent to the above-left corner ofthe target block. B₂ may be a block that occupies pixels located atcoordinates (xP−1, yP−1).

Determination of Availability of Spatial Candidate and TemporalCandidate

In order to include the motion information of a spatial candidate or themotion information of a temporal candidate in a list, it must bedetermined whether the motion information of the spatial candidate orthe motion information of the temporal candidate is available.

Hereinafter, a candidate block may include a spatial candidate and atemporal candidate.

For example, the determination may be performed by sequentially applyingthe following steps 1) to 4).

Step 1) When a PU including a candidate block is out of the boundary ofa picture, the availability of the candidate block may be set to“false”. The expression “availability is set to false” may have the samemeaning as “set to be unavailable”.

Step 2) When a PU including a candidate block is out of the boundary ofa slice, the availability of the candidate block may be set to “false”.When the target block and the candidate block are located in differentslices, the availability of the candidate block may be set to “false”.

Step 3) When a PU including a candidate block is out of the boundary ofa tile, the availability of the candidate block may be set to “false”.When the target block and the candidate block are located in differenttiles, the availability of the candidate block may be set to “false”.

Step 4) When the prediction mode of a PU including a candidate block isan intra-prediction mode, the availability of the candidate block may beset to “false”. When a PU including a candidate block does not use interprediction, the availability of the candidate block may be set to“false”.

FIG. 11 illustrates the order of addition of motion information ofspatial candidates to a merge list according to an embodiment.

As shown in FIG. 11, when pieces of motion information of spatialcandidates are added to a merge list, the order of A₁, B₁, B₀, A₀, andB₂ may be used. That is, pieces of motion information of availablespatial candidates may be added to the merge list in the order of A₁,B₁, B₀, A₀, and B₂.

Method for Deriving Merge List in Merge Mode and Skip Mode

As described above, the maximum number of merge candidates in the mergelist may be set. The set maximum number is indicated by “N”. The setnumber may be transmitted from the encoding apparatus 100 to thedecoding apparatus 200. The slice header of a slice may include N. Inother words, the maximum number of merge candidates in the merge listfor the target block of the slice may be set by the slice header. Forexample, the value of N may be basically 5.

Pieces of motion information (i.e., merge candidates) may be added tothe merge list in the order of the following steps 1) to 4).

Step 1) Among spatial candidates, available spatial candidates may beadded to the merge list. Pieces of motion information of the availablespatial candidates may be added to the merge list in the orderillustrated in FIG. 10. Here, when the motion information of anavailable spatial candidate overlaps other motion information alreadypresent in the merge list, the motion information may not be added tothe merge list. The operation of checking whether the correspondingmotion information overlaps other motion information present in the listmay be referred to in brief as an “overlap check”.

The maximum number of pieces of motion information that are added may beN.

Step 2) When the number of pieces of motion information in the mergelist is less than N and a temporal candidate is available, the motioninformation of the temporal candidate may be added to the merge list.Here, when the motion information of the available temporal candidateoverlaps other motion information already present in the merge list, themotion information may not be added to the merge list.

Step 3) When the number of pieces of motion information in the mergelist is less than N and the type of a target slice is “B”, combinedmotion information generated by combined bidirectional prediction(bi-prediction) may be added to the merge list.

The target slice may be a slice including a target block.

The combined motion information may be a combination of L0 motioninformation and L1 motion information. L0 motion information may bemotion information that refers only to a reference picture list L0. L1motion information may be motion information that refers only to areference picture list L1.

In the merge list, one or more pieces of L0 motion information may bepresent. Further, in the merge list, one or more pieces of L1 motioninformation may be present.

The combined motion information may include one or more pieces ofcombined motion information. When the combined motion information isgenerated, L0 motion information and L1 motion information, which are tobe used for generation, among the one or more pieces of L0 motioninformation and the one or more pieces of L1 motion information, may bepredefined. One or more pieces of combined motion information may begenerated in a predefined order via combined bidirectional prediction,which uses a pair of different pieces of motion information in the mergelist. One of the pair of different pieces of motion information may beL0 motion information and the other of the pair may be L1 motioninformation.

For example, combined motion information that is added with the highestpriority may be a combination of L0 motion information having a mergeindex of 0 and L1 motion information having a merge index of 1. Whenmotion information having a merge index of 0 is not L0 motioninformation or when motion information having a merge index of 1 is notL1 motion information, the combined motion information may be neithergenerated nor added. Next, the combined motion information that is addedwith the next priority may be a combination of L0 motion information,having a merge index of 1, and L1 motion information, having a mergeindex of 0. Subsequent detailed combinations may conform to othercombinations of video encoding/decoding fields.

Here, when the combined motion information overlaps other motioninformation already present in the merge list, the combined motioninformation may not be added to the merge list.

Step 4) When the number of pieces of motion information in the mergelist is less than N, motion information of a zero vector may be added tothe merge list.

The zero-vector motion information may be motion information for whichthe motion vector is a zero vector.

The number of pieces of zero-vector motion information may be one ormore. The reference picture indices of one or more pieces of zero-vectormotion information may be different from each other. For example, thevalue of the reference picture index of first zero-vector motioninformation may be 0. The value of the reference picture index of secondzero-vector motion information may be 1.

The number of pieces of zero-vector motion information may be identicalto the number of reference pictures in the reference picture list.

The reference direction of zero-vector motion information may bebidirectional. Both of the motion vectors may be zero vectors. Thenumber of pieces of zero-vector motion information may be the smallerone of the number of reference pictures in the reference picture list L0and the number of reference pictures in the reference picture list L1.Alternatively, when the number of reference pictures in the referencepicture list L0 and the number of reference pictures in the referencepicture list L1 are different from each other, a reference directionthat is unidirectional may be used for a reference picture index thatmay be applied only to a single reference picture list.

The encoding apparatus 100 and/or the decoding apparatus 200 maysequentially add the zero-vector motion information to the merge listwhile changing the reference picture index.

When zero-vector motion information overlaps other motion informationalready present in the merge list, the zero-vector motion informationmay not be added to the merge list.

The order of the above-described steps 1) to 4) is merely exemplary, andmay be changed. Further, some of the above steps may be omitteddepending on predefined conditions.

Method for Deriving Prediction Motion Vector Candidate List in AMVP Mode

The maximum number of prediction motion vector candidates in aprediction motion vector candidate list may be predefined. Thepredefined maximum number is indicated by N. For example, the predefinedmaximum number may be 2.

Pieces of motion information (i.e. prediction motion vector candidates)may be added to the prediction motion vector candidate list in the orderof the following steps 1) to 3).

Step 1) Available spatial candidates, among spatial candidates, may beadded to the prediction motion vector candidate list. The spatialcandidates may include a first spatial candidate and a second spatialcandidate.

The first spatial candidate may be one of A₀, A₁, scaled A₀, and scaledA₁. The second spatial candidate may be one of B₀, B₁, B₂, scaled B₀,scaled B₁, and scaled B₂.

Pieces of motion information of available spatial candidates may beadded to the prediction motion vector candidate list in the order of thefirst spatial candidate and the second spatial candidate. In this case,when the motion information of an available spatial candidate overlapsother motion information already present in the prediction motion vectorcandidate list, the motion information may not be added to theprediction motion vector candidate list. In other words, when the valueof N is 2, if the motion information of a second spatial candidate isidentical to the motion information of a first spatial candidate, themotion information of the second spatial candidate may not be added tothe prediction motion vector candidate list.

The maximum number of pieces of motion information that are added may beN.

Step 2) When the number of pieces of motion information in theprediction motion vector candidate list is less than N and a temporalcandidate is available, the motion information of the temporal candidatemay be added to the prediction motion vector candidate list. In thiscase, when the motion information of the available temporal candidateoverlaps other motion information already present in the predictionmotion vector candidate list, the motion information may not be added tothe prediction motion vector candidate list.

Step 3) When the number of pieces of motion information in theprediction motion vector candidate list is less than N, zero-vectormotion information may be added to the prediction motion vectorcandidate list.

The zero-vector motion information may include one or more pieces ofzero-vector motion information. The reference picture indices of the oneor more pieces of zero-vector motion information may be different fromeach other.

The encoding apparatus 100 and/or the decoding apparatus 200 maysequentially add pieces of zero-vector motion information to theprediction motion vector candidate list while changing the referencepicture index.

When zero-vector motion information overlaps other motion informationalready present in the prediction motion vector candidate list, thezero-vector motion information may not be added to the prediction motionvector candidate list.

The description of the zero-vector motion information, made above inconnection with the merge list, may also be applied to zero-vectormotion information. A repeated description thereof will be omitted.

The order of the above-described steps 1) to 3) is merely exemplary, andmay be changed. Further, some of the steps may be omitted depending onpredefined conditions.

FIG. 12 illustrates a transform and quantization process according to anexample.

As illustrated in FIG. 12, quantized levels may be generated byperforming a transform and/or quantization process on a residual signal.

A residual signal may be generated as the difference between an originalblock and a prediction block. Here, the prediction block may be a blockgenerated via intra prediction or inter prediction.

The residual signal may be transformed into a signal in a frequencydomain through a transform procedure that is a part of a quantizationprocedure.

A transform kernel used for a transform may include various DCT kernels,such as Discrete Cosine Transform (DCT) type 2 (DCT-II) and DiscreteSine Transform (DST) kernels.

These transform kernels may perform a separable transform or atwo-dimensional (2D) non-separable transform on the residual signal. Theseparable transform may be a transform indicating that a one-dimensional(1D) transform is performed on the residual signal in each of ahorizontal direction and a vertical direction.

The DCT type and the DST type, which are adaptively used for a 1Dtransform, may include DCT-V, DCT-VIII, DST-I, and DST-VII in additionto DCT-II, as shown in the following Table 3.

TABLE 3 Transform set Transform candidates 0 DST-VII, DCT-VIII 1DST-VII, DST-I 2 DST-VII, DCT-V

As shown in Table 3, when a DCT type or a DST type to be used for atransform is derived, transform sets may be used. Each transform set mayinclude multiple transform candidates. Each transform candidate may be aDCT type or a DST type.

The following Table 4 shows examples of a transform set that is appliedto a horizontal direction depending on the intra-prediction mode.

TABLE 4 Intra-prediction Transform mode set  0 2  1 1  2 0  3 1  4 0  51  6 0  7 1  8 0  9 1 10 0 11 1 12 0 13 1 14 2 15 2 16 2 17 2 18 2 19 220 2 21 2 22 2 23 1 24 0 25 1 26 0 27 1 28 0 29 1 30 0 31 1 32 0 33 1

In Table 4, the number of each transform set to be applied to thehorizontal direction of a residual signal is indicated depending on theintra-prediction mode of the target block.

The following Table 5 shows examples of a transform set that is appliedto the vertical direction of the residual signal depending on theintra-prediction mode.

TABLE 5 Intra-prediction mode Transform set  0 2  1 1  2 0  3 1  4 0  51  6 0  7 1  8 0  9 1 10 0 11 1 12 0 13 1 14 0 15 0 16 0 17 0 18 0 19 020 0 21 0 22 0 23 1 24 0 25 1 26 0 27 1 28 0 29 1 30 0 31 1 32 0 33 1

As exemplified in FIGS. 4 and 5, transform sets to be applied to thehorizontal direction and the vertical direction may be predefineddepending on the intra-prediction mode of the target block. The encodingapparatus 100 may perform a transform and an inverse transform on theresidual signal using a transform included in the transform setcorresponding to the intra-prediction mode of the target block. Further,the decoding apparatus 200 may perform an inverse transform on theresidual signal using a transform included in the transform setcorresponding to the intra-prediction mode of the target block.

In the transform and inverse transform, transform sets to be applied tothe residual signal may be determined, as exemplified in Tables 3, 4,and 5, and may not be signaled. Transform indication information may besignaled from the encoding apparatus 100 to the decoding apparatus 200.The transform indication information may be information indicating whichone of multiple transform candidates included in the transform set to beapplied to the residual signal is used.

As described above, methods using various transforms may be applied to aresidual signal generated via intra prediction or inter prediction.

The transform may include at least one of a first transform and asecondary transform. A transform coefficient may be generated byperforming the first transform on the residual signal, and a secondarytransform coefficient may be generated by performing the secondarytransform on the transform coefficient.

The first transform may be referred to as a “primary transform”.Further, the first transform may also be referred to as an “AdaptiveMultiple Transform (AMT) scheme”. AMT may mean that, as described above,different transforms are applied to respective 1D directions (i.e. avertical direction and/or a horizontal direction) or a selecteddirection.

Alternatively, an AMT may be referred to as a Multiple TransformSelection (MTS) or Extended Multiple Transform (EMT).

A secondary transform may be a transform for improving energyconcentration on a transform coefficient generated by the firsttransform. Similar to the first transform, the secondary transform maybe a separable transform or a non-separable transform. Such anon-separable transform may be a Non-Separable Secondary Transform(NSST).

The first transform may be performed using at least one of predefinedmultiple transform methods. For example, the predefined multipletransform methods may include a Discrete Cosine Transform (DCT), aDiscrete Sine Transform (DST), a Karhunen-Loeve Transform (KLT), etc.

Further, a first transform may be a transform having various typesdepending on a kernel function that defines a DCT or a DST.

For example, the first transform may include transforms, such as DCT-2,DCT-5, DCT-7, DST-1, and DST-8 depending on the transform kernelpresented in the following Table 6. In the following Table 6, varioustransform types and transform kernel functions for Multiple TransformSelection (MTS) are exemplified.

MTS may refer to the selection of combinations of one or more DCT and/orDST kernels so as to transform a residual signal in a horizontal and/orvertical direction.

TABLE 6 Transform type Transform kernel function T_(i)(j) DCT-2${T_{i}(j)} = {{\omega_{0} \cdot \sqrt{\frac{2}{N}} \cdot {\cos\left( \frac{\pi \cdot i \cdot \left( {{2j} + 1} \right)}{2N} \right)}}\mspace{14mu}{where}}$$\omega_{0} = {\sqrt{\frac{2}{N}}\left( {i = 0} \right)\mspace{14mu}{or}\mspace{14mu} 1\mspace{11mu}({otherwise})}$DST-7${T_{i}(j)} = {\sqrt{\frac{4}{{2N} + 1}} \cdot {\sin\left( \frac{\pi \cdot \left( {{2j} + 1} \right) \cdot \left( {j + 1} \right)}{{2N} + 1} \right)}}$DCT-5${T_{i}(j)} = {{\omega_{0} \cdot \omega_{1} \cdot \sqrt{\frac{2}{{2N} - 1}} \cdot {\cos\left( \frac{2{\pi \cdot i \cdot j}}{{2N} + 1} \right)}}\mspace{14mu}{where}}$$\omega_{0/1} = {\sqrt{\frac{2}{N}}\left( {{i\mspace{14mu}{or}\mspace{14mu} j} = 0} \right)\mspace{14mu}{or}\mspace{14mu} 1\mspace{11mu}({otherwise})}$DST-8${T_{i}(j)} = {\sqrt{\frac{4}{{2N} + 1}} \cdot {\cos\left( \frac{\pi \cdot \left( {{2j} + 1} \right) \cdot \left( {{2j} + 1} \right)}{{4N} + 2} \right)}}$DST-1${T_{i}(j)} = {\sqrt{\frac{2}{N + 1}} \cdot {\sin\left( \frac{\pi \cdot \left( {i + 1} \right) \cdot \left( {j + 1} \right)}{N + 1} \right)}}$

In Table 6, i and j may be integer values that are equal to or greaterthan 0 and are less than or equal to N−1.

The secondary transform may be performed on the transform coefficientgenerated by performing the first transform.

A first transform and/or a secondary transform may be applied to signalcomponents corresponding to one or more of a luminance (luma) componentand a chrominance (chroma) component. Whether to apply the firsttransform and/or the secondary transform may be determined depending onat least one of coding parameters for a target block and/or aneighboring block. For example, whether to apply the first transformand/or the secondary transform may be determined depending on the sizeand/or shape of the target block.

The transform method(s) to be applied to a first transform and/or asecondary transform may be determined depending on at least one ofcoding parameters for a target block and/or a neighboring block. Thedetermined transform method may also indicate that a first transformand/or a secondary transform are not used.

Alternatively, transform information indicating a transform method maybe signaled from the encoding apparatus 100 to the decoding apparatus200. For example, the transform information may include the index of atransform to be used for a first transform and/or a secondary transform.

The quantized transform coefficient (i.e. the quantized levels) may begenerated by performing quantization on the result, generated byperforming the primary transform and/or the secondary transform, or onthe residual signal.

FIG. 13 illustrates diagonal scanning according to an example.

FIG. 14 illustrates horizontal scanning according to an example.

FIG. 15 illustrates vertical scanning according to an example.

Quantized transform coefficients may be scanned via at least one of(up-right) diagonal scanning, vertical scanning, and horizontal scanningdepending on at least one of an intra-prediction mode, a block size, anda block shape. The block may be a Transform Unit (TU).

Each scanning may be initiated at a specific start point, and may beterminated at a specific end point.

For example, quantized transform coefficients may be changed to 1Dvector forms by scanning the coefficients of a block using diagonalscanning of FIG. 13. Alternatively, horizontal scanning of FIG. 14 orvertical scanning of FIG. 15, instead of diagonal scanning, may be useddepending on the size and/or intra-prediction mode of a block.

Vertical scanning may be the operation of scanning 2D block-typecoefficients in a column direction. Horizontal scanning may be theoperation of scanning 2D block-type coefficients in a row direction.

In other words, which one of diagonal scanning, vertical scanning, andhorizontal scanning is to be used may be determined depending on thesize and/or inter-prediction mode of the block.

As illustrated in FIGS. 13, 14, and 15, the quantized transformcoefficients may be scanned along a diagonal direction, a horizontaldirection or a vertical direction.

The quantized transform coefficients may be represented by block shapes.Each block may include multiple sub-blocks. Each sub-block may bedefined depending on a minimum block size or a minimum block shape.

In scanning, a scanning sequence depending on the type or direction ofscanning may be primarily applied to sub-blocks. Further, a scanningsequence depending on the direction of scanning may be applied toquantized transform coefficients in each sub-block.

For example, as illustrated in FIGS. 13, 14, and 15, when the size of atarget block is 8×8, quantized transform coefficients may be generatedthrough a primary transform, a secondary transform, and quantization onthe residual signal of the target block. Therefore, one of three typesof scanning sequences may be applied to four 4×4 sub-blocks, andquantized transform coefficients may also be scanned for each 4×4sub-block depending on the scanning sequence.

The scanned quantized transform coefficients may be entropy-encoded, anda bitstream may include the entropy-encoded quantized transformcoefficients.

The decoding apparatus 200 may generate quantized transform coefficientsvia entropy decoding on the bitstream. The quantized transformcoefficients may be aligned in the form of a 2D block via inversescanning. Here, as the method of inverse scanning, at least one ofup-right diagonal scanning, vertical scanning, and horizontal scanningmay be performed.

In the decoding apparatus 200, dequantization may be performed on thequantized transform coefficients. A secondary inverse transform may beperformed on the result generated by performing dequantization dependingon whether to perform the secondary inverse transform. Further, a firstinverse transform may be performed on the result generated by performingthe secondary inverse transform depending on whether the first inversetransform is to be performed. A reconstructed residual signal may begenerated by performing the first inverse transform on the resultgenerated by performing the secondary inverse transform.

FIG. 16 is a configuration diagram of an encoding apparatus according toan embodiment.

An encoding apparatus 1600 may correspond to the above-describedencoding apparatus 100.

The encoding apparatus 1600 may include a processing unit 1610, memory1630, a user interface (UI) input device 1650, a UI output device 1660,and storage 1640, which communicate with each other through a bus 1690.The encoding apparatus 1600 may further include a communication unit1620 coupled to a network 1699.

The processing unit 1610 may be a Central Processing Unit (CPU) or asemiconductor device for executing processing instructions stored in thememory 1630 or the storage 1640. The processing unit 1610 may be atleast one hardware processor.

The processing unit 1610 may generate and process signals, data orinformation that are input to the encoding apparatus 1600, are outputfrom the encoding apparatus 1600, or are used in the encoding apparatus1600, and may perform examination, comparison, determination, etc.related to the signals, data or information. In other words, inembodiments, the generation and processing of data or information andexamination, comparison and determination related to data or informationmay be performed by the processing unit 1610.

The processing unit 1610 may include an inter-prediction unit 110, anintra-prediction unit 120, a switch 115, a subtractor 125, a transformunit 130, a quantization unit 140, an entropy encoding unit 150, adequantization unit 160, an inverse transform unit 170, an adder 175, afilter unit 180, and a reference picture buffer 190.

At least some of the inter-prediction unit 110, the intra-predictionunit 120, the switch 115, the subtractor 125, the transform unit 130,the quantization unit 140, the entropy encoding unit 150, thedequantization unit 160, the inverse transform unit 170, the adder 175,the filter unit 180, and the reference picture buffer 190 may be programmodules, and may communicate with an external device or system. Theprogram modules may be included in the encoding apparatus 1600 in theform of an operating system, an application program module, or otherprogram modules.

The program modules may be physically stored in various types ofwell-known storage devices. Further, at least some of the programmodules may also be stored in a remote storage device that is capable ofcommunicating with the encoding apparatus 1200.

The program modules may include, but are not limited to, a routine, asubroutine, a program, an object, a component, and a data structure forperforming functions or operations according to an embodiment or forimplementing abstract data types according to an embodiment.

The program modules may be implemented using instructions or codeexecuted by at least one processor of the encoding apparatus 1600.

The processing unit 1610 may execute instructions or code in theinter-prediction unit 110, the intra-prediction unit 120, the switch115, the subtractor 125, the transform unit 130, the quantization unit140, the entropy encoding unit 150, the dequantization unit 160, theinverse transform unit 170, the adder 175, the filter unit 180, and thereference picture buffer 190.

A storage unit may denote the memory 1630 and/or the storage 1640. Eachof the memory 1630 and the storage 1640 may be any of various types ofvolatile or nonvolatile storage media. For example, the memory 1630 mayinclude at least one of Read-Only Memory (ROM) 1631 and Random AccessMemory (RAM) 1632.

The storage unit may store data or information used for the operation ofthe encoding apparatus 1600. In an embodiment, the data or informationof the encoding apparatus 1600 may be stored in the storage unit.

For example, the storage unit may store pictures, blocks, lists, motioninformation, inter-prediction information, bitstreams, etc.

The encoding apparatus 1600 may be implemented in a computer systemincluding a computer-readable storage medium.

The storage medium may store at least one module required for theoperation of the encoding apparatus 1600. The memory 1630 may store atleast one module, and may be configured such that the at least onemodule is executed by the processing unit 1610.

Functions related to communication of the data or information of theencoding apparatus 1600 may be performed through the communication unit1220.

For example, the communication unit 1620 may transmit a bitstream to adecoding apparatus 1600, which will be described later.

FIG. 17 is a configuration diagram of a decoding apparatus according toan embodiment.

The decoding apparatus 1700 may correspond to the above-describeddecoding apparatus 200.

The decoding apparatus 1700 may include a processing unit 1710, memory1730, a user interface (UI) input device 1750, a UI output device 1760,and storage 1740, which communicate with each other through a bus 1790.The decoding apparatus 1700 may further include a communication unit1720 coupled to a network 1399.

The processing unit 1710 may be a Central Processing Unit (CPU) or asemiconductor device for executing processing instructions stored in thememory 1730 or the storage 1740. The processing unit 1710 may be atleast one hardware processor.

The processing unit 1710 may generate and process signals, data orinformation that are input to the decoding apparatus 1700, are outputfrom the decoding apparatus 1700, or are used in the decoding apparatus1700, and may perform examination, comparison, determination, etc.related to the signals, data or information. In other words, inembodiments, the generation and processing of data or information andexamination, comparison and determination related to data or informationmay be performed by the processing unit 1710.

The processing unit 1710 may include an entropy decoding unit 210, adequantization unit 220, an inverse transform unit 230, anintra-prediction unit 240, an inter-prediction unit 250, a switch 245,an adder 255, a filter unit 260, and a reference picture buffer 270.

At least some of the entropy decoding unit 210, the dequantization unit220, the inverse transform unit 230, the intra-prediction unit 240, theinter-prediction unit 250, the adder 255, the switch 245, the filterunit 260, and the reference picture buffer 270 of the decoding apparatus200 may be program modules, and may communicate with an external deviceor system. The program modules may be included in the decoding apparatus1700 in the form of an operating system, an application program module,or other program modules.

The program modules may be physically stored in various types ofwell-known storage devices. Further, at least some of the programmodules may also be stored in a remote storage device that is capable ofcommunicating with the decoding apparatus 1700.

The program modules may include, but are not limited to, a routine, asubroutine, a program, an object, a component, and a data structure forperforming functions or operations according to an embodiment or forimplementing abstract data types according to an embodiment.

The program modules may be implemented using instructions or codeexecuted by at least one processor of the decoding apparatus 1700.

The processing unit 1710 may execute instructions or code in the entropydecoding unit 210, the dequantization unit 220, the inverse transformunit 230, the intra-prediction unit 240, the inter-prediction unit 250,the switch 245, the adder 255, the filter unit 260, and the referencepicture buffer 270.

A storage unit may denote the memory 1730 and/or the storage 1740. Eachof the memory 1730 and the storage 1740 may be any of various types ofvolatile or nonvolatile storage media. For example, the memory 1730 mayinclude at least one of ROM 1731 and RAM 1732.

The storage unit may store data or information used for the operation ofthe decoding apparatus 1700. In an embodiment, the data or informationof the decoding apparatus 1700 may be stored in the storage unit.

For example, the storage unit may store pictures, blocks, lists, motioninformation, inter-prediction information, bitstreams, etc.

The decoding apparatus 1700 may be implemented in a computer systemincluding a computer-readable storage medium.

The storage medium may store at least one module required for theoperation of the decoding apparatus 1700. The memory 1730 may store atleast one module, and may be configured such that the at least onemodule is executed by the processing unit 1710.

Functions related to communication of the data or information of thedecoding apparatus 1700 may be performed through the communication unit1720.

For example, the communication unit 1720 may receive a bitstream fromthe encoding apparatus 1600.

Image-Processing Method Using Sharing of Information Between Channels

A method and apparatus according to embodiments may applytransform-coding (transcoding) technology that uses prediction andvarious transforms to high-resolution images, such as 4K or 8Kresolution images, may encode and/or decode images by sharing varioustypes of predefined coding decision information between channels, andmay decode a compressed bitstream or compressed data for encoded imagesby sharing transmitted coding decision information between channels.

Multiple channels may mean multiple components representing a block. Forexample, the multiple channels may include a color channel, a depthchannel, an alpha channel, etc.

Hereinafter, the terms “channel” and “color” may have the same meaning,and may be used interchangeably with each other. Further, the term“color” may indicate one of channels. The term “channel” may be usedinterchangeably with one or more of the terms “color”, “depth”, and“alpha”.

Technology in the present embodiment may be used, and thus the problemof deteriorated compressibility and image quality, which occurs whenconventional technology is applied to encoding and decoding of images,can be solved. In particular, when conventional technology is applied toimages in which variation in pixel values is spatially concentrated, theproblem of the deterioration of compressibility and image quality may besevere.

In an example, as various types of coding decision information that areshared between channels to perform encoding and decoding according to anembodiment, the following pieces of information are present. In thenames of the following information, “flag” may be omitted.

1) Transform skip flag (transform_skip_flag) information may indicatewhether to selectively skip a transform. Alternatively, thetransform_skip_flag information may indicate one of the use of atransform and the skipping of a transform.

2) Intra smoothing filtering information may indicate whether smoothingfiltering is applied to reference pixels that are used inintra-prediction.

3) Position-Dependent intra-Prediction Combination (PDPC) flag(PDPC_flag) information may indicate whether intra-prediction is to beperformed by using neighbor pixels to which smoothing (i.e. filtering)is applied and neighbor pixels to which smoothing (i.e. filtering) isnot applied together when specific intra-prediction (e.g. planarprediction) is performed.

4) Residual Differential Pulse Coded Modulation (RDPCM) flag(rdpcm_flag) information may indicate whether RDPCM of additionallyperforming Differential Pulse Coded Modulation (DPCM) on a residualsignal, acquired via one prediction, and of again acquiring a residualsignal is to be performed.

5) Multiple Transform Selection (MTS) flag (mts_flag) information mayindicate whether an encoding method based on an Extended MultipleTransform (EMT) is to be used.

EMT may be an encoding method that selects and uses a specifiedtransform for a transform block, which is a target block among providedmultiple transforms.

EMT may also stand for “enhanced multiple transform”, and may alsoindicate “Multiple Transform Selection (MTS).

6) EMT flag information may indicate whether EMT is to be used.

7) MTS index (mts_idx) information may indicate which transforms are tobe used in a horizontal direction and a vertical direction when MTS isused.

A part of the mts_idx information (e.g. one specified bit in mts_idx)may be information indicating the transform that is used in thehorizontal direction of a residual signal.

Another part of the mts_idx information or a part of the remainder ofthe mts_idx information (e.g. one other specified bit in mts_idx) may beinformation indicating a transform that is used in the verticaldirection of a residual signal.

For example, the decision of a transform depending on the mts_idxinformation may be configured as shown in the following Tables 7 and 8.

TABLE 7 Transform in Transform Intra-prediction mts_idx horizontal invertical mode information direction direction 1 0 DCT-7 DCT-7 1 DCT-5 2DCT-7 DCT-5 3 DCT-5 1, 3, 5, 7, 9, 11, 13, 23, 0 DCT-7 DCT-7 25, 27, 29,31, 33, 35, 1 DCT-1 39, 41, 43, 45, 55, 57, 2 DCT-7 DCT-1 59, 61, 63, 653 DCT-1 2, 4, 6, 8, 10, 12, 24, 0 DCT-7 DCT-7 26, 28, 30, 32, 34, 36, 1DCT-8 38, 40, 42, 44, 56, 58, 2 DCT-7 DCT-8 60, 64, 66 3 DCT-8 14, 15,16, 17, 18, 19, 0 DCT-7 DCT-7 20, 21, 22 1 DCT-5 (angles neighboring 2DCT-7 DCT-8 horizontal directions) 3 DCT-5 46, 47, 48, 49, 50, 51, 0DCT-7 DCT-7 52, 53, 54 (angles 1 DCT-8 neighboring vertical 2 DCT-7DCT-5 directions) 3 DCT-8

TABLE 8 MTS_ MTS_ MTS_ Intra/inter CU_flag Hor_flag Ver_flag HorizontalVertical 0 DCT-2 1 0 0 DST-7 DST-7 0 1 DCT-8 DST-7 1 0 DST-7 DCT-8 1 1DCT-8 DCT-8

In Table 7, the transforms in a horizontal direction and the transformsin a vertical direction that are used depending on the values of theintra-prediction mode and mts_idx information are exemplified.

In accordance with Table 7, when mts_idx information is acquired so asto perform inter-prediction or inter-prediction of a target block, atransform in a horizontal direction and a transform in a verticaldirection that are to be used for the transform of the target block maybe decided on depending on the value of the mts_idx information.

For example, when the intra-prediction mode of the target block is 6 andthe value of the mts_idx information is 2, DCT-7 may be used as thetransform in a horizontal direction, and DCT-8 may be used as thetransform in a vertical direction.

Table 8 illustrates an example of a modification of Table 7. In Table 8,MTS_CU_flag may indicate that flag information mts_flag, indicatingwhether a Multiple Transform Selection (MTS) method is used, is decidedon and transmitted on a CU basis. Also, MTS_Hor_flag and MTS_Ver_flagmay respectively indicate a transform used in a horizontal direction anda transform used in a vertical direction. Table 8 may exemplifytransforms used in the horizontal direction and the vertical directionthrough the values of MTS_Hor_flag and MTS_Ver_flag.

Alternatively, the decision of transforms depending on the mts_idxinformation of Table 7 may be configured as shown in the following Table9.

TABLE 9 Intra-prediction Inter-prediction Horizontal Vertical HorizontalVertical mts_idx transform transform transform transform informationtype type type type 0 (00) 1 1 2 2 1 (01) 2 1 1 2 2 (10) 1 2 2 1 3 (11)2 2 1 1

In Table 9, in intra-prediction and inter-prediction, the horizontaltransform types and the vertical transform types that are used dependingon the values of the mts_idx information are exemplified.

Respective values of the horizontal transform types may indicatespecific transforms. For example, the value “1” of a horizontaltransform type may represent DST-7. The value “2” of a horizontaltransform type may represent DCT-8.

8) Non-Separable Secondary Transform (NSST) flag (nsst_flag) informationmay indicate whether an NSST encoding method for additionally performinga non-separable secondary transform on all or some transformcoefficients acquired via a primary transform is to be used.

9) NSST index (nsst_idx) information may indicate the type of secondarytransform to be applied to all or some transform coefficients when theNSST encoding method is used.

The nsst_idx information may indicate the transform to be used for anon-separable secondary transform.

10) CU skip flag information may indicate whether the transmission ofencoded data about a CU is to be skipped.

11) CU Local Illumination Compensation (LIC) flag (CU_lic_flag)information may indicate whether to compensate for the differencebetween the brightness values of blocks.

12) Overlapped Block Motion Compensation (OBMC) flag (obmc_flag)information may indicate whether to generate the final motioncompensation block using multiple overlapped motion compensation blocks.

13) codeAlfCtuEnable flag (codeAlfCtuEnable_flag) information mayindicate whether an Adaptive Loop Filter (ALF) is applicable to thepixel value of the current CTU.

When such coding decision information is shared between channels, imageshaving excellent image quality may be obtained while imagecompressibility is increased.

Describing the coding decision information to be shared between channelsaccording to the present embodiment, transform_skip_flag information maybe used as an example of the coding decision information to be sharedbetween channels, for convenience of the entire description andunderstanding of the embodiment, such as the description of operations,drawings, and equations.

However, the transform_skip_flag information is only a single example,and the coding decision information to be shared between channels towhich the present embodiment is applied does not necessarily mean onlythe transform_skip_flag information.

For example, it should be understood that one or more of theabove-described pieces of coding decision information, such as 1)rdpcm_flag information, 2) pieces of transform-related selectioninformation such as mts_flag information, mts_idx information, nsst_flaginformation, and nsst_idx information, 3) obmc_flag information, and 4)PDPC_flag information, which are required for decoding, are included inthe coding decision information to be shared between channels.

Also, when channels between which coding decision information requiredfor decoding is to be shared according to the embodiment are described,a YCbCr color space may be used as an example. However, the YCbCr colorspace is only a single detailed example, and embodiments may be appliedto various color spaces, such as a YUV color space, an XYZ color space,and an RGB color space.

A color index cIDX may be a channel index indicating one of channels ina color space.

For the YCbCr color space and the YUV color space, cIDX may have valuessuch as “0/1/2” for channels sequentially displayed in the correspondingcolor space. The values “a/b/c” may represent that the value of cIDXindicating a first channel is ‘a’, the value of cIDX indicating a secondchannel is ‘b’, and the value of cIDX indicating a third channel is ‘c’.

Alternatively, for the YCbCr color space and the YUV color space, cIDXmay have values such as “0/2/1” for channels sequentially displayed inthe corresponding color space.

For the RGB color space and the XYZ color space, cIDX may have valuessuch as “1/0/2” or “2/0/1” for the channels sequentially displayed inthe corresponding color space.

As image-compression technologies that have been developed or are beingdeveloped for the purpose of realizing high-efficiency imageencoding/decoding, there may be various technologies, such as 1)inter-prediction technology of predicting the value of a pixel includedin a target picture from a picture previous to or subsequent to thetarget picture, 2) intra-prediction technology of predicting the valueof a pixel included in the current target picture using information ofpixels in the target picture, 3) transform and quantization technologyof compressing the energy of a residual signal remaining as a predictionerror, 4) entropy-coding technology of assigning a short codeword tomore frequently appearing values and assigns a long codeword to lessfrequently appearing values, and arithmetic coding technology. Byutilizing these image-compression technologies, image data may beeffectively compressed, transmitted, and stored.

There is a great variety of compression technologies that can be appliedto encoding of images. Further, depending on the properties of theimages to be encoded, a specific compression technology may be moreprofitable than other compression technologies. Therefore, an encodingapparatus 1600 may perform the most profitable compression on a targetblock by adaptively deciding whether to use any of various types ofmultiple compression technologies for the target block.

Therefore, in order to select the compression technology most profitablefor the target block from among various selectable compressiontechnologies, the encoding apparatus 1600 may typically performRate-Distortion Optimization (RDO). Which one of various image encodingdecisions that can be selected for encoding of images is optimal may notbe known in advance from the standpoint of rate-distortion. Therefore,the encoding apparatus 1600 may calculate rate-distortion values forcombinations of all available image encoding decisions by performingencoding (or simplified encoding) on respective combinations of allavailable image encoding decisions, and may decide on and use an imageencoding decision having the smallest rate-distortion value, among thecalculated rate-distortion values, as the final image encoding decisionfor the target block.

Also, the encoding apparatus 1600 may record an encoding decision,derived by performing such RDO or derived using an additional decisionmethod selected by the encoding apparatus 1600, in a bitstream. Adecoding apparatus 1700 may read (i.e. parse) the encoding decisionrecorded in the bitstream, and may accurately perform decoding on thetarget block by performing a reverse process corresponding to encodingdepending on the encoding decision.

Here, information indicating encoding decision may be referred to as“coding decision information” or “coding information” required fordecoding.

Hereinafter, the terms “coding decision information” and “codinginformation” may have the same meaning, and may be used interchangeablywith each other.

Generally, multiple channels for images (e.g. YUV, YCbCr, RGB, and XYZ)may not always have identical or similar properties. Therefore, from thestandpoint of improvement of compressibility, making encoding decisionsindependently of each of multiple channels may generally realize betterperformance.

For example, as one of the above-described encoding decisions, there istransform_skip_flag, which is an encoding decision indicating whether toperform a transform on the target block. That is, whether a transform isto be skipped for each of blocks may be decided on, andtransform_skip_flag information indicating such a decision may berecorded as coding decision information in a bitstream for each ofmultiple channels.

Generally, in encoding for image compression, it has been consideredthat a transform for a target block is always performed. However, when aspatial change in the values of pixels in a target block that is thetarget of compression is very large, or especially when a change in thepixel values is very locally limited, the extent to which image energyis concentrated on low frequencies may not be great even if a transformis applied, and instead, a larger number of transform coefficients for ahigh-frequency area having relatively large values may occur.

Therefore, when low-frequency signal components are mostly maintainedand high-frequency signal components are eliminated by means of atransform and quantization process, or when a transform and quantizationprocess for reducing the amount of data by applying strong quantizationis applied, serious degradation of image quality may occur. Inparticular, when a spatial change in the values of pixels is very largeor which a change in pixel values is concentrated on a very locallylimited region, such degradation of image quality may be furtherincreased.

In order to solve the above-described problems, a method for directlyencoding the values of pixels in a spatial domain without a transform,instead of uniformly applying a transform to a target block, may beused. In accordance with this method, whether to perform a transform oneach transform block may be decided on. By performing or skipping atransform based on such a decision, encoding of the transform block maybe performed. In a bitstream, transform_skip_flag information, which iscoding decision information indicating whether the performance of atransform is to be skipped, may be recorded.

For example, when the value of the transform_skip_flag information is 1,a transform may be skipped. When the value of the transform_skip_flaginformation is 0, a transform may be performed. The encoding apparatus1600 may transfer information about whether a transform is to be skippedfor the target block to the decoding apparatus 1700 through thetransform_skip_flag information, and the above-described problems may besolved by means of this transfer.

Also, pieces of transform_skip_flag information may be respectively setfor a luma channel (i.e. Y channel) and chroma channels (i.e. Cb channeland Cr channel), and may then be transmitted. The decoding apparatus1700 may perform decoding on the target block by skipping or performinga transform on the channel of the target block depending on the value ofthe transform_skip_flag information for each channel, which is read(i.e., parsed) from the bitstream.

However, when pieces of transform_skip_flag information for channelssuch as Y, Cb, and Cr are transmitted for all transform blocks,additional problems may arise in that overhead may increase due tosignaling of pieces of transform_skip_flag information, and thecompressibility of images may be deteriorated.

In order to mitigate problems such as deterioration of compressibility,flag information indicating whether a transform is to be skipped may belimitedly transmitted only when the size of a transform block is lessthan or equal to a specific transform block size. However, even thoughthis scheme is used, pieces of flag information indicating whether atransform is to be skipped for all channels must be transmitted for eachtransform block having a size greater than the specific block size,which may still deteriorate the compressibility of images. Also, suchdeteriorated compressibility inevitably decreases the quality ofcompressed images.

In order to solve the deterioration of compressibility caused bytransmitting pieces of coding decision information, selected by theencoding apparatus 1600, for all channels, encoding and/or decodingmethods that use the sharing of information between channels aredisclosed in embodiments.

First, conditions according to which image properties of channels aredetermined to be similar to each other may be predefined. When theseconditions are satisfied, coding decision information for an image or ablock determined by the encoding apparatus 1600 for a representative oneof multiple channels may be transmitted to the decoding apparatus 1700.

For all of the remaining channels other than the representative channelor a channel selected from among the remaining channels, among themultiple channels, coding decision information that is transferred forthe representative channel may be shared and used. By means of thissharing and usage, the compressibility of images may be improved.Therefore, the encoding and/or decoding methods according to the presentembodiment may provide excellent encoding efficiency even if respectivepieces of coding decision information for multiple channels are nottransmitted.

Here, the coding decision information to be shared may include one ormore of the above-described transform_skip_flag information,intra-smoothing filtering information, rdpcm_flag information, mts_flaginformation, mts_idx information, PDPC_flag information, MTS_CU_flaginformation, MTS_Hor_flag information, MTS_Ver_flag information,nsst_flag information, nsst_idx information, CU skip flag information,CU_lic_flag information, obmc_flag information, codeAlfCtuEnable_flaginformation, and PDPC_flag information.

Conditions Under which Image Properties of Respective Channels areDetermined to be Similar to Each Other

In order to determine that the image properties of channels are similarto each other, whether cross-channel prediction (inter-channelprediction) has been used for a target block may be checked.

That is, in order to predict the decoding target channel of the targetblock, whether to use a prediction method for acquiring a predictionvalue for the decoding target channel by applying a specific model tothe reconstructed information of an additional channel (e.g. a lumachannel) may be checked. For example, the reconstructed information maybe the pixel value of a reconstructed pixel or the value of atransformation coefficient. The specified model may be a linear model.

A decoding target channel may be the channel that is the target to becurrently decoded, among multiple channels. An encoding target channelmay be the channel that is the target to be currently encoded, amongmultiple channels. Hereinafter, the encoding target channel and/or thedecoding target channel may be simply referred to as a “target channel”.

For example, whether intra-prediction for the target block uses anintra-prediction mode that derives a prediction value for the targetchannel by utilizing reconstructed information of an additional channelmay be checked.

In order to derive the prediction value for the target channel using thereconstructed information of an additional channel, a Cross-ComponentLinear Model (CCLM) that uses a single linear model, a Multi-Mode LinearModel (MMLM) that uses multiple linear models, and a multifilter linearmodel that uses multiple filters may be used. In the CCLM, the term“component” may be replaced with “channel”.

The INTRA_CCLM mode may be an intra-prediction mode that uses a CCLM.The INTRA_MMLM mode may be an intra-prediction mode that uses an MMLM.The INTRA_MFLM mode may be an intra-prediction mode that uses an MFLM.

Alternatively, a determination that the image properties of channels aresimilar to each other may be implemented by checking whether theintra-prediction mode of the target block (e.g. intra_chroma_pred_modeinformation indicating an intra-prediction mode for the chroma channelof the target block) is one of an INTRA_CCLM mode, an INTRA_MMLM mode,and an INTRA_MFLM mode.

Alternatively, a determination that the image properties of channels aresimilar to each other may be implemented by checking whether theencoding mode of a target channel of the target block uses the encodingmode of an additional channel (e.g. a luma channel) without change. Forexample, a determination that the image properties of channels aresimilar to each other may be implemented by checking whether theintra-prediction mode of the target block (e.g. intra_chroma_pred_modeinformation) is a direct mode (DM). The direct mode may also be referredto as a “derived mode”. DM may be a mode indicating that theintra-prediction mode of a luma channel is used as the intra-predictionmode of a chroma channel without change due to the characteristicwhereby the correlation between the luma channel and the chroma channelmay be higher.

The features of the DM, which is one of intra-prediction modes, and thedetailed operation thereof may be defined in greater detail withreference to the following Table 10 and Table 11.

Table 10 shows a method for setting an IntraPredModeC value forintra-prediction of a chroma signal (when the value ofsps_cclm_enabled_flag information is 0).

Table 11 shows a method for setting an IntraPredModeC value forintra-prediction of a chroma signal (when the value ofsps_cclm_enabled_flag information is 1).

TABLE 10 IntraPredModeY intra_chroma_ X ( 0 < = pred_mode 0 50 18 1 X <= 66 ) 0 66  0  0  0  0 1 50 66 50 50 50 2 18 18 66 18 18 3  1  1  1 66 1 4  0 50 18  1 X

TABLE 11 IntraPredModeY intra_chroma_ X 0 < = pred_mode 0 50 18 1 X < =66) 0 66  0  0  0  0 1 50 66 50 50 50 2 18 18 66 18 18 3  1  1  1 66  14 77 77 77 77 77 5  0 50 18  1 X

Generally, in intra-prediction, it may be determined whether to use anINTRA_Cross-Component Linear Model (CCLM) mode, an INTRA_Multi-Model LM(MMLM) mode or an INTRA_Multi-Filter LM (MFLM) mode in which the pixelvalue of a reconstructed pixel for a single channel (e.g. a lumachannel, or more generally, a representative channel) is used tocalculate a prediction value for an additional channel (e.g. a chromachannel, or more generally, a target channel).

Indication of the case where the INTRA_CCLM mode, the INTRA_MMLM mode orthe INTRA_MFLM mode is used may be classified into two types and definedin detail depending on the value of sps_cclm_enabled_flag information,as shown in Table 10 and Table 11.

The sps_cclm_enabled_flag information may be information indicatingwhether the INTRA_CCLM mode, the INTRA_MMLM mode, and the INTRA_MFLMmode are to be enabled. Alternatively, the sps_cclm_enabled_flaginformation may be information indicating whether the INTRA_CCLM mode,the INTRA_MMLM mode, and the INTRA_MFLM mode have been enabled.

When the value of the sps_cclm_enabled_flag is 0, the INTRA_CCLM mode,the INTRA_MMLM mode, and the INTRA_MFLM mode may not be used, whereaswhen the value of the sps_cclm_enabled_flag is 1, the INTRA_CCLM modemay be used. Alternatively, when the value of the sps_cclm_enabled_flagis 1, at least one of the INTRA_MMLM mode and the INTRA_MFLM mode may beused.

Whether the intra-prediction mode (e.g. intra_chroma_pred_modeinformation) of the target channel is a DM may be determined by checkingwhether the intra-prediction mode (i.e., the value of theintra_chroma_pred_mode) of the chroma channel is a specific value (e.g.4 in Table 10 and 5 in Table 11). In the description of this operation,when the value of the sps_cclm_enabled_flag is 0, Table 10 may bereferred to, whereas when the value of the sps_cclm_enabled_flag is 1,Table 11 may be referred to.

When the value of the sps_cclm_enabled_flag is 0 and the value of theintra-prediction mode (e.g., intra_chroma_pred_mode) of the targetchannel is 4, it may be considered that the DM is applied.Alternatively, when the value of the sps_cclm_enabled_flag is 1 and thevalue of the intra-prediction mode (e.g., intra_chroma_pred_mode) of thetarget channel is 5, it may be considered that the DM is applied. Forintra-prediction of the target channel (e.g. a chroma channel) of thetarget block indicated by the DM, the value of IntraPredModeY,indicating the intra-prediction mode of a representative channel (e.g. aluma channel), may be used as the value of IntraPredModeC withoutchange.

Here, the intra-prediction mode intra_chroma_pred_mode of the chromasignal may be index information indicating which type ofintra-prediction is to be used for the chroma signal.

By means of this index information, the final value indicating theintra-prediction mode that is actually used for intra-prediction of thechroma signal may be the value of IntraPredModeC. In other words,IntraPredModeC may indicate the intra-prediction mode that is actuallyused for intra-prediction of the chroma signal.

When the value of sps_cclm_enabled_flag is 0 and the DM is applied (i.e.the value of the intra_chroma_pred_mode is 4), if the value ofIntraPredModeY is 0, 50, 18 or 1, the value of IntraPredModeC may alsobe 0, 50, 18 or 1.

Here, a value of 0 may mean a planar mode (i.e. planar prediction orplanar direction), a value of 1 may mean a DC mode, a value of 18 maymean a horizontal mode, a value of 50 may mean a vertical mode, and avalue of 66 may mean a diagonal mode.

When the value of IntraPredModeY is an additional value X different fromany of the four values 0, 50, 18, and 1, the value of IntraPredModeC atthat time may also be X, equal to the value of IntraPredModeY.

Meanwhile, when the value of cclm_enabled_flag is 0, if the value ofIntraPredModeY is 0, 50, 18 or 1, the value of IntraPredModeC may bedecided on depending on the value of IntraPredModeY, as shown in thefirst four rows in Table 10.

For example, as described in the first row in Table 10, when the valueof IntraPredModeY is 0, 50, 18 or 1, the value of IntraPredModeC may be66, 0, 0 or 0. When the value of IntraPredModeY is an additional valueother than 0, 50, 18, and 1, the value of IntraPredModeC may be 0.

Further, when the value of sps_cclm_enabled_flag is 1 and the DM isapplied (i.e. the value of the intra_chroma_pred_mode is 5), if thevalue of IntraPredModeY is 0, 50, 18 or 1, the value of IntraPredModeCmay also be 0, 50, 18 or 1.

Here, a value of 0 may mean a planar mode (i.e. planar prediction orplanar direction), a value of 1 may mean a DC mode, a value of 18 maymean a horizontal mode, a value of 50 may mean a vertical mode, and avalue of 66 may mean a diagonal mode.

When the value of IntraPredModeY is an additional value X different fromany of the four values 0, 50, 18, and 1, the value of IntraPredModeC atthat time may also be X, equal to the value of IntraPredModeY.

Meanwhile, when the value of cclm_enabled_flag is 1, if the value ofIntraPredModeY is 0, 50, 18 or 1, the value of IntraPredModeC may bedecided on depending on the value of IntraPredModeY, as shown in thefirst five rows in Table 11.

For example, as described in the first row in Table 11, when the valueof IntraPredModeY is 0, 50, 18 or 1, the value of IntraPredModeC may be66, 0, 0 or 0. When the value of IntraPredModeY is an additional valueother than 0, 50, 18, and 1, the value of IntraPredModeC may be 0.

In a further embodiment, determination that the image properties ofchannels are similar to each other may be implemented by checkingwhether, as an encoding mode for the target channel of the target block,a mode, indicating that only a specific mode limited by the encodingmode of an additional channel (e.g. a luma channel) is to be used, isused. For example, a determination that the image properties of channelsare similar to each other may be made by checking whether theintra-prediction mode of the target channel is a Direct Mode (DM).

Cross-Channel Prediction Using Correlation Between Channels

Cross-channel prediction may be technology that uses the pixel value ofa pixel in an additional channel rather than using intra-prediction orinter-prediction when predicting the pixel value of a pixel in a targetchannel.

When a target block is encoded, the fact that the performance ofcross-channel prediction is better than that of other types ofprediction may mean that considerable similarity is present between thepixel values of pixels in the channels of the target block.

Therefore, in this case, when the value of coding decision informationof a representative channel is determined, it may be profitable toequally use the determined value of the coding decision information ofthe representative channel for the coding decision information of anadditional channel or to use a specific value indicated by the value ofthe coding decision information of the representative channel for thecoding decision information of the additional channel.

For example, when the value of transform_skip_flag information of therepresentative channel is 0, indicating that a transform is not skipped,the probability that the determined value of transform_skip_flaginformation will be ‘0’ may be high even in other channels.

Therefore, for an image or a block for which cross-channel prediction iseffective, there may occur the case where there is no need toindividually designate pieces of transform_skip_flag information formultiple channels. The reason for this is that the similarity betweenchannels is high, and thus the probability that pieces oftransform_skip_flag information of respective channels will be identicalto each other may be high.

Although such image properties are present, compressibility and imagequality may be deteriorated when pieces of transform_skip_flaginformation are respectively transmitted for channels of the image.

This principle may also be applied to additional coding decisioninformation, that is, mts_flag information, mts_idx information,nsst_flag information, nsst_idx information, intra-smoothing filteringinformation, PDPC_flag information, and rdpcm_flag information, and theprobability that the value of coding decision information for therepresentative channel will be identical to that of the coding decisioninformation for an additional channel may be high.

Therefore, based on the conditions under which the image properties ofthe channels are determined to be similar to each other, whethercross-channel prediction using a correlation between channels has beendecided on as the encoding mode of the target block may be determined.

For example, the determination of whether cross-channel prediction hasbeen decided on as the encoding mode of the target block may be intendedto determine whether the image properties of channels are similar toeach other depending on whether a Color-Component Linear prediction Mode(CCLM), indicating cross-channel prediction is applied to the targetblock. In order to determine whether the CCLM is applied to the targetblock, whether the prediction mode of the target block is one of anINTRA_CCLM mode, an INTRA_MMLM mode, and an INTRA_MFLM mode may bechecked.

For example, the determination of whether the cross-channel predictionhas been decided on as the encoding mode of the target block may beintended to determine whether the intra mode of the representativechannel (e.g. a luma channel) is used for an additional channel (e.g.chroma channels Cb and Cr) without change. Alternatively, thedetermination of whether cross-channel prediction has been decided on asthe encoding mode of the target block may be intended to determinewhether a specific intra mode, indicated by the intra mode of therepresentative channel, is used in an additional channel. Alternatively,the determination of whether cross-channel prediction has been decidedon as the encoding mode of the target block may be intended to determinewhether a specific intra mode, derived from the intra mode of therepresentative channel, is used for an additional channel.

For example, the determination of whether cross-channel prediction hasbeen decided on as the encoding mode of the target block may be intendedto determine whether the encoding apparatus 1600 and the decodingapparatus 1700 use a specific encoding mode (e.g. an inter-channelsharing mode) in agreement with each other.

For example, the determination of whether cross-channel prediction hasbeen decided on as the encoding mode of the target block may be intendedto determine whether the intra-prediction mode of a chroma channel is aDM.

Such a DM may be a mode indicating that, due to the characteristic inwhich the correlation between a luma channel and chroma channels may behigh, the intra-prediction mode of the chroma channels is used as theintra-prediction mode of the luma channel without change. Therefore,when the intra-prediction mode of the chroma channels of the targetblock is a DM, it may be determined that the conditions under which theimage properties of channels are determined to be similar to each otherare satisfied.

In addition to the conditions described in the above examples, whethercross-channel prediction has been decided on as the encoding mode of thetarget block may be determined based on the size of a block.

For example, the greater the size of the block, the higher theprobability that a pixel having a heterogeneous property will be presentin the corresponding block. Therefore, the similarity between thechannels of the block having a larger size may be less than that of ablock having a smaller size. Also, when the size of a block isexcessively small, the similarity between channels of the block may notbe stable.

For example, the sharing of information between channels may beperformed only for a block having a size that is less than or equal to aspecific size. The specific size may be 64×64, 32×32 or 16×16. When theblock size is less than or equal to the specific size, the sharing ofinformation between channels is performed, and thus the conditions underwhich the image properties of channels are determined to be similar toeach other may be more reliably satisfied.

Alternatively, the sharing of information between channels may beperformed only for a block having a size greater than a specific size.The specific size may be 4×4. When the size of the block is greater thanthe specific size, the sharing of information between channels isperformed, and thus the conditions under which the image properties ofchannels are determined to be similar to each other may be more reliablysatisfied.

Alternatively, only when the size of a block is greater than a firstspecific size (e.g. 4×4) and is less than or equal to a second specificsize (e.g. 32×32 or 64×64) may the sharing of information betweenchannels be performed. Only when the size of the block falls within aspecific range, may the sharing of information between channels beperformed, and thus the conditions under which the image properties ofchannels are determined to be similar to each other may be more reliablysatisfied.

In an embodiment, a method and apparatus for encoding a target blockthrough the sharing of information between channels will be describedbelow, and the following functions may be provided.

-   -   Coding decision information of one channel of a target block may        be parsed from a compressed bitstream, and decoding for all        channels or some selected channels of the target block may be        performed using the coding decision information of the one        channel.    -   A bitstream may be configured such that coding decision        information is transmitted only for a representative channel or        some selected channels of the target block.    -   Whether a transform is to be skipped for one channel may be        determined, and the determination of whether a transform is to        be skipped may be applied to additional channels.    -   For one channel of a transform block, transform_skip_flag        information may be parsed from the compressed bitstream. Whether        a transform is to be skipped may be determined for one channel        or multiple channels of the transform block by utilizing the        parsed transform_skip_flag information.    -   For one channel of the transform block, the transform_skip_flag        information may be signaled. The transform_skip_flag information        for one channel may be used even for additional channels.    -   Coding decision information may be efficiently signaled through        the sharing of information between channels. By means of this        efficient signaling, encoding efficiency and subjective image        quality may be improved.    -   In particular, when a spatial change in pixel values in a block        is very large or very sharp, the extent to which image energy is        concentrated on low frequencies may not be large even if a        transform is applied to the target block. Also, when        low-frequency signal components are mostly maintained and        high-frequency signal components are eliminated by applying a        transform and quantization process to such a block, or when        strong quantization is applied to such a block, serious        degradation of image quality may occur. In an embodiment,        whether a transform for a block is to be skipped may be        economically indicated based on the determination by the        encoding apparatus 1600 without causing large overhead. Through        such economical indication, the compressibility of images may be        improved, and the deterioration of image quality may be        minimized.    -   When cross-channel prediction technology that uses a correlation        between the channels is used, multiple pieces of coding decision        information may not be respectively used for multiple channels.        In an embodiment, coding decision information may be transmitted        for one channel, and the coding decision information transmitted        for the one channel may be shared and used with all of the        remaining channels or some channels selected from among the        remaining channels. By means of this sharing, the problem of the        deterioration of compressibility and image quality may be        solved.

Decision of Representative Channel Depending on Color Space

As color spaces for image encoding and decoding, there are YCbCr and YUVspaces, which are used for encoding and decoding of a general image, andin addition, RGB, XYZ, and YCoCg spaces are present. When one of variouscolor spaces is a target color space that is used for encoding anddecoding of an image, one of the channels of the target color space maybe decided on as a representative channel of the target color space.

In an embodiment, the color channel having the highest correlation witha luma signal, among the channels, may be decided on as therepresentative channel. For example, in an RGB color space, a G channelmay have the highest correlation with the luma signal, and thus the Gchannel may be selected as the representative channel. In an XYZ colorspace, a Y channel may have the highest correlation with the lumasignal, and thus the Y channel may be selected as the representativechannel. In a YCoCg color space, a Y channel may have the highestcorrelation with the luma signal, and thus the Y channel may be selectedas the representative channel.

The channels in the color space may be represented by index values suchas “0/1/2”. SelectedCIDX may be the index of a selected color.Alternatively, SelectedCIDX may be an index value indicating theselected representative channel. The representative channel may bedecided on by the index SelectedCIDX indicating a selectedrepresentative channel, among pieces of information about a target blockin a bitstream.

For example, in a YCbCr color space, the value of SelectedCIDX may be 0,indicating a Y channel.

For example, in the YCbCr color space, a Cb channel may be decided on asthe representative channel. When the Cb channel is decided on as therepresentative channel, the value of SelectedCIDX may be 1, indicatingthe Cb channel.

For example, in a YUV color space, a U channel may be decided on as therepresentative channel. When the U channel is decided on as therepresentative channel, the value of SelectedCIDX may be 1, indicatingthe U channel.

For the sharing of the coding decision information between channels, aspecific channel in the color space may be selected as a representativechannel. In the encoding and decoding of an image, the coding decisioninformation of the representative channel may be shared between one ormore remaining channels.

For example, the encoding apparatus 1600 may signal only the codingdecision information of the representative channel to the decodingapparatus 1700 through a bitstream. Alternatively, the decodingapparatus 1700 may derive the coding decision information of therepresentative channel using the bitstream. The coding decisioninformation of at least some of the remaining channels may not beseparately signaled. The decoding apparatus 1700 may derive the codingdecision information of at least some of the remaining channels usingthe coding decision information of the representative channel. In otherwords, the coding decision information of the representative channel maybe shared with at least some of the remaining channels.

For example, when the Y channel, having the highest correlation with theluma signal, is selected as the representative channel in the YCbCrcolor space, a correlation may be present between a luma channel (i.e.Y) and a chroma channel (i.e. Cb and/or Cr). Therefore, when predictionis performed for image compression, coding decision information for theluma channel, which is the representative channel, may be implicitlyshared as pieces of coding decision information for one or more chromablocks, instead of respectively applying independent predictions tothree channels in the color space. The one or more chroma blocks mayinclude one or more of a Cb block and a Cr block.

For example, when the Cb channel is selected as the representativechannel in the YCbCr color space, a correlation may be present between aCb signal and a Cr signal constituting the chroma channels. Therefore,when prediction is performed for image compression, coding decisioninformation for the Cb channel, which is the representative channel, maybe implicitly shared as coding decision information for the Cr block,instead of respectively applying independent predictions to two chromachannels.

Coding Decision Information Shared Between Channels

The coding decision information that can be shared between channels maybe information, such as a syntax element, which is encoded by theencoding apparatus 1600 and which is signaled to the decoding apparatus1700 as information contained in a bitstream. For example, the codingdecision information may include a flag, an index, etc. Also, the codingdecision information may include information derived during an encodingand/or decoding process. Further, the coding decision information maymean information required for encoding and/or decoding an image.

For example, the coding decision information may include at least one orcombinations of the size of a unit/block, the depth of the unit/block,the partition information of the unit/block, the partition structure ofthe unit/block, partition flag information indicating whether theunit/block is partitioned in the form of a quad tree, partition flaginformation indicating whether the unit/block is partitioned in the formof a binary tree, the partitioning direction of a binary tree form(horizontal direction or vertical direction), the partitioning form of abinary tree form (symmetrical partitioning or asymmetricalpartitioning), partition flag information indicating whether theunit/block is partitioned in the form of a tertiary tree, thepartitioning direction of the tertiary tree form (horizontal directionor vertical direction), the partitioning form of the tertiary tree form(symmetrical partitioning or asymmetrical partitioning), informationindicating whether the unit/block is partitioned in the form of acomplex tree, the combination and direction of partitioning of a complextree form (horizontal direction or vertical direction), a predictionscheme (intra-prediction or inter-prediction), an intra-predictionmode/direction, a reference sample filtering method, a prediction blockfiltering method, a prediction-block boundary filtering method, thefilter tap of filtering, the filter coefficient of filtering, aninter-prediction mode, motion information, a motion vector, a referencepicture index, an inter-prediction direction, an inter-predictionindicator, a reference picture list, a reference image, a motion vectorpredictor, a motion vector prediction candidate, a motion vectorcandidate list, information indicating whether a merge mode is used, amerge candidate, a merge candidate list, information indicating whethera skip mode is used, the type of an interpolation filter, the filter tapof the interpolation filter, the filter coefficient of the interpolationfilter, the magnitude of a motion vector, precision of representation ofa motion vector, a transform type, a transform size, informationindicating whether a primary transform is used, information indicatingwhether an additional (secondary) transform is used, primary transformselection information (or a primary transform index), secondarytransform selection information (or a secondary transform index),information indicating the presence or absence of a residual signal, acoded block pattern, a coded block flag, a quantization parameter, aquantization matrix, information about an intra-loop filter, informationabout whether an intra-loop filter is applied, the coefficient of theintra-loop filter, the filter tap of the intra-loop filter, theshape/form of the intra-loop filter, information indicating whether adeblocking filter is applied, the coefficient of the deblocking filter,the tap of the deblocking filter, the strength of the deblocking filter,the shape/form of the deblocking filter, information indicating whetheran adaptive sample offset is applied, information indicating whether anadaptive sample offset is applied, the value of an adaptive sampleoffset, the category of the adaptive sample offset, the type of theadaptive sample offset, information indicating whether an adaptivein-loop filter is applied, the coefficient of the adaptive in-loopfilter, the tap of the adaptive in-loop filter, the shape/form of theadaptive in-loop filter, a binarization/debinarization method, a contextmodel, a context model decision method, a context model update method,information indicating whether a regular mode is performed, informationindicating whether a bypass mode is performed, a context bin, a bypassbin, a transform coefficient, a transform coefficient level, a transformcoefficient level scanning method, an image display/output sequence,slice identification information, a slice type, slice partitioninformation, tile identification information, tile type information,tile partition information, a picture type, bit depth, information abouta luma signal and information about a chroma signal, transform_skip_flaginformation, primary transform selection information, secondarytransform selection information, reference sample filtering information,PDPC_flag information, rdpcm_flag information, EMT flag information,mts_flag information, mts_idx information, nsst_flag information, andnsst_idx information.

Among pieces of coding decision information that can be shared betweenchannels, primary transform selection information may be transforminformation required in order to perform a transform procedure on aresidual signal using a combination of one or more DCT transform kernelsand/or DST transform kernels related to a horizontal direction and/or avertical direction. For example, the primary transform selectioninformation may be information required in order to use MTS in theprimary transform. The primary transform selection information mayinclude mts_flag information and mts_idx information.

The primary transform selection information applied to the target blockmay be explicitly signaled, or alternatively may be implicitly derivedby the encoding apparatus 1600 and the decoding apparatus 1700 using thecoding decision information of the target block and the coding decisioninformation of a neighbor block.

After the primary transform has been completed in the encoding apparatus1600, a secondary transform may be performed so as to improve the energyconcentration of transform coefficients.

The secondary transform selection information that is applied to thetarget block may be explicitly signaled, or alternatively may beimplicitly derived by the encoding apparatus 1600 and the decodingapparatus 1700 using the code decision information of the target blockand the coding decision information of a neighbor block. The decodingapparatus 1700 may perform a secondary inverse transform depending onwhether the secondary inverse transform is to be performed, and mayperform a primary inverse transform on the results of performing thesecondary inverse transform depending on whether a primary inversetransform is to be performed.

The encoding apparatus 1600 may generate rdpcm_flag information for thetarget block, and may record the rdpcm_flag information in a bitstream.The decoding apparatus 1700 may acquire the rdpcm_flag informationthrough the bitstream, and may perform RDPCM depending on informationindicated by the rdpcm_flag information, or may not perform RDPCM.

FIG. 18 is a flowchart of a method for decoding coding decisioninformation according to an embodiment.

In accordance with embodiments, when a specific channel in a color spaceis selected as a representative channel so as to share informationbetween multiple channels, the coding decision information of therepresentative channel of the target block may be shared by one or moreremaining channels other than the representative channel, among themultiple channels.

For example, when intra-prediction for the target block is performed ina YCbCr color space, a Y channel may be set as a representative channel,and thereafter the intra-coding decision information of therepresentative channel may be shared and used to perform decoding ofchannels other than the representative channel (i.e. a Cb channel and/ora Cr channel), instead of respectively, independently transmittingpieces of intra-coding decision information for three channels in thecolor space. Alternatively, after the Cb channel is assumed to be arepresentative channel in the YCbCr color space, the intra-codingdecision information of the representative channel may be implicitlyused for the decoding of the Cr channel, which is a channel other thanthe representative channel.

For example, the intra-coding decision information of the representativechannel that can be shared by the remaining channels may include one ormore of an intra-prediction mode, an intra-prediction direction, aprediction-block boundary filtering method, a filter tap forprediction-block boundary filtering, a filter coefficient forprediction-block boundary filtering, transform_skip_flag information,primary transform selection information, secondary transform selectioninformation, mts_flag information, mts_idx information, PDPC_flaginformation, rdpcm_flag information, EMT flag information, nsst_flaginformation, nsst_idx information, intra-smoothing filteringinformation, CU skip flag information, CU_lic_flag information,obmc_flag information, codeAlfCtuEnable_flag information, and PDPC_flaginformation.

The case where cross-channel prediction is selected from among varioustechnologies including angular prediction, DC prediction, planarprediction, etc. for predicting a chroma signal or where cross-channelprediction is more profitable may mean that the properties of a lumasignal (i.e. a Y signal) are considerably similar to those of a chromasignal (i.e. Cb and/or Cr signals).

In this case, when the channel of a Y signal block is a representativechannel, coding decision information, determined in the process ofencoding and/or decoding the representative channel, may be equallyapplied to a chroma block. By means of this application, the number ofbits required in order to transmit the coding decision information maybe reduced. Therefore, encoding and decoding may be performed so that asingle piece of coding decision information is used for multiplechannels via cross-channel prediction.

For example, after the coding decision information for a luma channelhas been determined, the determined coding decision information may beshared between the remaining channels, and encoding may be performedbased on the shared coding decision information. Alternatively, piecesof coding decision information may be independently applied to threechannels without being shared between the channels, and the threechannels may be independently encoded and/or decoded. The encodingapparatus 1600 may decide on a method that is profitable from thestandpoint of rate-distortion, among methods for sharing coding decisioninformation between the channels and methods for independently encodingthe channels, as an encoding method. Depending on this decision, theencoding apparatus 1600 may explicitly write information about whethercoding decision information is shared between channels in a bitstream,and this information may be transmitted to the decoding apparatus 1700through the bitstream.

For example, when a specific encoding condition is satisfied in theencoding and/or decoding process, the information about whether codingdecision information is shared between channels may not be explicitlysignaled, and the coding decision information of a representativechannel may be shared by the remaining channels.

For example, the specific encoding condition may be a conditionindicating whether Cross-Component Prediction (CCP), a DM or aCross-Component Linear Model (CCLM) is used.

For example, when the remaining chroma signal (i.e. a Cb signal and/or aCr signal) is predicted using at least one of an original signal, areconstructed signal, a residual signal, and a prediction signal of theluma signal for which encoding or decoding has been completed, thesharing of coding decision information between channels may be applied.

For example, when a luma signal is predicted using at least one of anoriginal signal, a reconstructed signal, a residual signal, and aprediction signal of the chroma signal for which encoding or decodinghas been completed, the sharing of coding decision information betweenchannels may be applied.

For example, when a Cr signal is predicted using at least one of anoriginal signal, a reconstructed signal, a residual signal, and aprediction signal of a Cb signal for which encoding or decoding has beencompleted, the sharing of coding decision information between channelsmay be applied.

For example, when a Cb signal is predicted using at least one of anoriginal signal, a reconstructed signal, a residual signal, and aprediction signal of the Cr signal for which encoding or decoding hasbeen completed, the sharing of coding decision information betweenchannels may be applied.

When the luma channel is set as a representative channel in a YCbCrcolor space, coding decision information may be signaled only for theluma signal, and coding decision information may not be separatelytransmitted for the remaining chroma channels, instead of being signaledto the remaining channels other than the luma channel. Through suchselective transmission, compressibility may be improved.

Further, coding decision information may be transmitted only for the Cbsignal and the transmitted coding decision information may be shared forthe Cr signal, and thus compressibility may be improved. Alternatively,coding decision information may be transmitted only for the Cr signaland the transmitted coding decision information may be shared for the Cbsignal, and thus compressibility may be improved.

At step 1810, a communication unit 1720 may receive a bitstream. Thebitstream may include coding decision information.

At step 1820, a processing unit 1710 may determine whether the sharingof coding decision information is to be used for a target channel of atarget block.

When coding decision information is not to be shared, step 1830 may beperformed.

When coding decision information is to be shared, step 1840 may beperformed.

At step 1830, the processing unit 1710 may acquire the coding decisioninformation of the target channel from the bitstream. The processingunit 1710 may parse and read the coding decision information of thetarget channel from the bitstream.

At step 1840, the processing unit 1710 may set the coding decisioninformation so that the coding decision information of therepresentative channel is used as the coding decision information of thetarget channel.

Steps 1820, 1830, and 1840 may be represented by the following Code 1:

[Code 1] if ((cIdx != 0) && “cross-channel prediction is used”)  codingdecision information [cIdx] = coding decision information [0] else PARSE coding decision information [cIdx] from compressed bitstream

cIdx may indicate the target channel of the target block. For example,when the number of channels in a target image is 3, cIdx may be one ofpredefined specific values, and may be one of {0, 1, 2}.

In an embodiment, the cIdx of the representative channel may be assumedto be 0.

“cIdx !=0” may indicate that the target channel is not a representativechannel (e.g. a luma channel). In other words, a cIdx value of “0” mayindicate that the target channel is a representative channel.

In other words, at step 1820, when the target channel is not arepresentative channel and cross-channel prediction is used for thetarget block, the processing unit 1710 may determine to use the sharingof coding decision information for the target channel. When the targetchannel is a representative channel or when cross-channel prediction isnot used for the target block, the processing unit 1710 may determinenot to use the sharing of coding decision information for the targetchannel.

Whether cross-channel prediction is used for the target block may be 1)derived based on information acquired from the bitstream and 2)implicitly derived depending on whether a specific condition issatisfied.

As described above, whether cross-channel prediction is used may bedetermined based on the intra-prediction mode of the target block.Whether cross-channel prediction is used may be determined based onwhether the intra-prediction mode of the target block is one of anINTRA_CCLM mode, an INTRA_MMLM mode, and an INTRA_MFLM mode. Forexample, the processing unit 1710 may determine that cross-channelprediction is used when the intra-prediction mode of the target block isone of the INTRA_CCLM mode, the INTRA_MMLM mode, and the INTRA_MFLMmode.

As described above, whether cross-channel prediction is used may bedetermined based on whether the intra-prediction mode of the chromachannel of the target block has a specific value. For example, when theintra-prediction mode of the chroma channel of the target block has aspecific value, the processing unit 1710 may determine thatcross-channel prediction is used.

The intra-prediction mode of the chroma channel of the target block maybe indicated by intra_chroma_pred_mode information.

As described above, whether cross-channel prediction is used may bedetermined based on whether the intra-prediction mode of the targetchannel is a DM. For example, when the intra-prediction mode of thetarget channel is a DM, the processing unit 1710 may determine thatcross-channel prediction is used.

At step 1830, the processing unit 1710 may acquire the coding decisioninformation of a block of the channel indicated by cIdx from thebitstream.

The processing unit 1710 may parse and read the coding decisioninformation of the block of the channel indicated by cIdx from thebitstream.

At step 1840, the processing unit 1710 may share the coding decisioninformation of the representative channel as coding decision informationof the block of the channel indicated by cIdx. In other words, theprocessing unit 1710 may set the coding decision information of therepresentative channel as coding decision information of the block ofthe channel indicated by cIdx.

In accordance with an embodiment, an operation corresponding to acondition or an execution may be additionally performed before step 1820or between steps 1820 and 1840.

In accordance with an embodiment, for a Cb signal, coding decisioninformation may be transmitted, and for a Cr signal, the coding decisioninformation of the Cb signal may be shared without transmission ofcoding decision information. In this case, the above-described code 1may be modified to the following Code 2:

[Code 2] if ((cIdx != 1) && “cross-channel prediction is used”)  codingdecision information [cIdx] = coding decision information[0] else PARSEcoding decision information [cIdx] from compressed bitstream

FIG. 19 is a flowchart of a decoding method for determining whether atransform is to be skipped according to an embodiment.

An encoding apparatus 1600 may determine whether a transform (e.g. aprimary transform and/or a secondary transform) is to be skippeddepending on the size of a target block.

The target block may be a transform block.

log 2TrafoSize may denote the size of the target block.

For example, the encoding apparatus 1600 may skip the transform of thetarget block when the size of the target block is less than or equal toa threshold value indicating the boundary value of a block size.

Log 2MaxTransformSkipSize may denote a threshold value indicating theboundary value of the block size.

When the transform of the target block is skipped, the encodingapparatus 1600 may set the value of transform_skip_flag information to 1without performing a transform. The transform_skip_flag information maybe transmitted to the decoding apparatus 1700 through a bitstream.

Also, when the transform of the target block is performed, the encodingapparatus 1600 may perform a transform, and may set the value of thetransform_skip_flag information to 0. The transform_skip_flaginformation may be transmitted to the decoding apparatus 1700 throughthe bitstream.

Here, pieces of transform_skip_flag information may be separatelytransmitted for channels constituting the color space of an image.

The decoding apparatus 1700 may acquire the value of thetransform_skip_flag information from the bitstream. In other words, thedecoding apparatus 1700 may parse and read the transform_skip_flaginformation from the bitstream.

Here, the decoding apparatus 1700 may acquire the value of thetransform_skip_flag information from the bitstream only when the size ofa block is less than or equal to the threshold value indicating theboundary value of the block size.

Also, the decoding apparatus 1700 may acquire pieces oftransform_skip_flag information for multiple channels of an image.

The acquisition of the transform_skip_flag information may berepresented by the following Code 3:

  [Code 3] If ( log2TrafoSize <= Log2MaxTransformSkipSize ) transform_skip_flag[ x0 ][ y0 ][ cIdx ]

x0 and y0 may denote spatial coordinates indicating the location of thetarget block.

cIdx may indicate the target channel of target block information.

When there are three image channels, cIdx may have one of the predefinedvalues {0, 1, 2}. The value of a representative channel may be 0.

Code 3 may be modified to the following Code 4:

    [Code 4]   If  ((  log2TbWidth   <=   Log2MaxTransformSkipSize_W  )  && ( log2TbHeight <= Log2MaxTransformSkipSize_H ))   transform_skip_flag[ x0 ][ y0 ][ cIdx ]

log 2TbWidth may be a value based on the following Equation 11. “width”may be the width of the target block (i.e. horizontal length thereof).

log 2TbWidth=log₂width  [Equation 11]

log 2TbHeight may have a value based on the following Equation 12.“height” may be the height of the target block (i.e. the vertical lengththereof).

log 2TbHeight=log₂height  [Equation 12]

Predefined threshold values Log 2MaxTransformSkipSize_W and Log2MaxTransformSkipSize_H may be equal to each other or may be differentfrom each other. For example, the value of Log 2MaxTransformSkipSize_Wmay be 2, and the value of Log 2MaxTransformSkipSize_H may be 2.

Code 3 may be modified to the following Code 5:

  [Code 5] If (( log2TbWidth <= 2) && ( log2TbHeight <= 2 )) transform_skip_flag[ x0 ][ y0 ][ cIdx ]

As described above, instead of pieces of transform_skip_flag informationbeing separately signaled for multiple channels, transform_skip_flaginformation may be signaled only for a luma (Y) signal, andtransform_skip_flag information may not be separately signaled for theremaining chroma channels. Alternatively, transform_skip_flaginformation may be signaled only for the Cb signal, and may not beseparately signaled for the Cr signal, and the transform_skip_flaginformation transmitted for the Cb signal may be shared for the Crsignal.

Below, an embodiment in which the transform_skip_flag information isshared will be described.

In the embodiment, the decoding apparatus 1700 may acquire thetransform_skip_flag information from a bitstream. This acquisition maybe represented by the following Code 6:

  [Code 6] if ( log2TrafoSize <= Log2MaxTransformSkipSize ) {  if ((cIdx != 0) && “cross-channel prediction is used”)  transform_skip_flag[x0 ][ y0 ][cIdx] = transform_skip_flag[ x0 ][ y0 ][0]  else  PARSEtransform_skip_flag[ x0 ][ y0 ][cIdx] from compressed bitstream } else transform_skip_flag[ x0 ][ y0 ][cIdx] = 0

x0 and y0 may be spatial coordinates indicating the location of a targetblock.

cIdx may indicate the target channel of the target block.

In Code 6 and other codes including the condition “if (log 2TrafoSizeLog 2MaxTransformSkipSize)”, the condition “if (log 2TrafoSize<=Log2MaxTransformSkipSize)” may be replaced with the condition “if ((log2TbWidth<=Log 2MaxTransformSkipSize_W) && (log 2TbHeight<=Log2MaxTransformSkipSize_H))” or the condition “if ((log 2TbWidth<=2) &&(log 2TbHeight<=2))”.

When the number of channels of the image is 3, cIdx may have one of thepredefined values {0, 1, 2}. The value of the representative channel maybe 0. Alternatively, the value of the representative channel may be 1 or2.

At step 1910, a communication unit 1720 may receive a bitstream.

At step 1920, the processing unit 1710 may determine whether it ispossible to skip a transform for the target block.

If it is determined that it is possible to skip a transform, step 1930may be performed.

If it is determined that it is not possible to skip a transform, step1960 may be performed.

For example, when that the size of the target block is less than orequal to a specific size, the processing unit 1710 may determine that itis not possible to skip a transform.

For example, when the size of the target block is greater than thespecific size, the processing unit 1710 may determine that it is notpossible to skip a transform.

Here, the specific size may be a boundary value for the block size forwhich it is permitted to skip a transform.

For example, when the condition in the following Code 7 is satisfied(i.e., when the result of the condition in Code 7 is true), theprocessing unit 1710 may determine that it is not possible to skip atransform, whereas when the condition in the following Code 7 is notsatisfied (i.e. when the result of the condition in Code 7 is false),the processing unit 1710 may determine that it is possible to skip atransform.

if (log 2TrafoSize<=Log 2MaxTransformSkipSize)  [Code 7]

At step 1930, the processing unit 1710 may determine whether the sharingof transform_skip_flag information with the target channel of the targetblock is to be used.

If it is determined that the transform_skip_flag information is not tobe shared, step 1940 may be performed.

If it is determined that the transform_skip_flag information is to beshared, step 1950 may be performed.

For example, when the condition in the following Code 8 is satisfied(i.e. when the result of the condition in Code 8 is true), theprocessing unit 1710 may determine that the transform_skip_flaginformation is to be shared, whereas when the condition in the followingCode 8 is not satisfied (i.e. when the result of the condition in Code 8is false), the processing unit 1710 may determine that thetransform_skip_flag information is not to be shared.

if ((cIdx !=0)&&(cross-channel prediction is used))  [Code 8]

In other words, at step 1930, when the target channel is not arepresentative channel and cross-channel prediction is used for thetarget block, the processing unit 1710 may determine that thetransform_skip_flag information is to be shared with the target channel.In contrast, when the target channel is the representative channel orwhen cross-channel prediction is not used for the target block, theprocessing unit 1710 may determine that the transform_skip_flaginformation is not to be shared.

At step 1940, the processing unit 1710 may acquire transform_skip_flaginformation of the target channel from the bitstream. The processingunit 1710 may parse and read the transform_skip_flag information of thetarget channel from the bitstream.

The transform_skip_flag information may be stored intransform_skip_flag[x0][y0][cIdx].

At step 1950, the processing unit 1710 may set transform_skip_flaginformation so that the transform_skip_flag information of therepresentative channel is used as the transform_skip_flag information ofthe target channel.

The processing unit 1710 may use the transform_skip_flag information ofthe representative channel as the transform_skip_flag information of thetarget channel instead of parsing and reading the transform_skip_flaginformation of the target channel from the bitstream. In other words,the processing unit 1710 may store the value of transform_skip_flag[x0][y0][0] in transform_skip_flag[x0][y0][cIdx].

That is, without requiring a procedure for parsing and reading thetransform_skip_flag information of the target channel from thebitstream, the value previously stored in transform_skip_flag [x Offy0][0] may be equally used in transform_skip_flag[x0][y0][cIdx].

For example, transform_skip_flag information signaled for the luma (Y)channel, which is the representative channel, may be used even for thechroma channel (Cb and/or Cr).

In accordance with an embodiment, for the Cb signal, transform_skip_flaginformation may be transmitted, and for the Cr signal, thetransform_skip_flag information for the Cb signal may be shared withouttransmission of transform_skip_flag information. In this case, theabove-described Code 6 may be modified to the following Code 9:

    [Code 9]   if ( log2TrafoSize <= Log2MaxTransformSkipSize ) {    if((cIdx != 1) && “cross-channel prediction is used”)   transform_skip_flag[   x0   ][   y0   ][   cIdx   ]   =transform_skip_flag[ x0 ][ y0 ][ 1 ]    else    PARSEtransform_skip_flag[ x0 ][ y0 ][ cIdx ] from compressed bitstream   }else    transform_skip_flag[ x0 ][ y0 ][ cIdx ] = 0

When it is not possible to skip a transform for the target block, step1960 may be performed.

At step 1960, information indicating that a transform is not to beskipped for the target block may be set. Because skipping a transformfor the target block is not permitted, the value of transform_skip_flag[x0][y0][cIdx] may be set to 0.

FIG. 20 is a flowchart of a decoding method for determining whether atransform is to be skipped with reference to an intra mode according toan embodiment.

There may be a considerable correlation between the luma channel (i.e.Y) channel and the chroma channel (i.e. Cb and/or Cr) of an image. Forexample, the luma channel may include a large amount of informationabout the texture of the image, and the Cb channel and the Cr channel,which are chroma channels, may additionally provide color information tobe added to the texture.

Therefore, when prediction required for compression and reconstructionof an image is performed, prediction values for the Cb block and the Crblock for which prediction is performed from the signal of the lumachannel, previously acquired through decoding, may be calculated insteadof independent predictions being respectively performed for threechannels of a color space.

Technology for calculating these prediction values may be referred to as“Cross-Channel Prediction (CCP)” or the above-described “CCLM”.

The decoding apparatus 1700 may determine whether cross-channelprediction has been used by checking whether the intra-prediction modeof the target block is one of an INTRA_CCLM mode, an INTRA_MMLM mode,and an INTRA_MFLM mode.

Since a considerable portion of the texture information of the chromasignal is also included in the luma signal, such cross-channelprediction may be effective. Similarly, prediction values for the Crblock, which is the target of prediction, may be calculated from thesignal of the Cb channel using cross-channel prediction.

The case where cross-channel prediction is selected from among varioustechnologies including angular prediction, DC prediction, and planarprediction for the prediction of the chroma signal or wherecross-channel prediction is profitable may mean that the signalcharacteristics of the channel corresponding to SelectedCIDX areconsiderably similar to the signal characteristics of an additionalchannel.

Therefore, when it is profitable to skip (or perform) a transform forthe block of the channel corresponding to SelectedCIDX, it may beequally profitable to also skip (or perform) a transform for the blocksof the remaining channels.

Therefore, when cross-channel prediction is used, the parsing of abitstream may not be respectively performed for three channels in orderto acquire transform_skip_flag information. When the transform_skip_flaginformation of the representative channel is parsed, thetransform_skip_flag information of the remaining channels may not beseparately parsed. The transform_skip_flag information of therepresentative channel may be shared and used as the transform_skip_flaginformation of the remaining channels, and information indicating suchsharing may be recorded in a bitstream. For example, for such sharing,whether a transform is to be skipped may be determined using a channelcorresponding to SelectedCIDX.

Alternatively, the rate-distortion values may be calculated for the casewhere a transform is equally skipped for three channels, and therate-distortion values may be calculated for the case where a transformis equally performed for the three channels. The rate-distortion valuescalculated when the transform is skipped and the rate-distortion valuescalculated when the transform is performed may be compared with eachother, and the encoding of channels may be performed using a moreprofitable scheme between a scheme for skipping a transform and a schemefor performing a transform based on the result of the comparison.

In an embodiment, instead of pieces of transform_skip_flag informationbeing signaled for multiple channels, transform_skip_flag informationmay be signaled only for the channel corresponding to SelectedCIDX, andtransform_skip_flag information may not be separately signaled for theremaining channels.

Below, an embodiment in which such transform_skip_flag information isshared will be described.

In an embodiment, the decoding apparatus 1700 may acquiretransform_skip_flag information from the bitstream. Such acquisition maybe represented by the following Code 10:

    [Code 10]   if ( log2TrafoSize <= Log2MaxTransformSkipSize ) {   if( (cIdx != SelectedCIDX) && “cross-channel prediction is used”)   transform_skip_flag[   x  0 ][   y0   ][   cIdx   ]   =transform_skip_flag[ x0 ][ y0 ][SelectedCIDX]    else    PARSEtransform_skip_flag[ x0 ][ y0 ][ cIdx ] from compressed bitstream   }else    transform_skip_flag[ x0 ][ y0 ][ cIdx] = 0

x0 and y0 may be spatial coordinates indicating the location of thetarget block.

cIdx may indicate the target channel of the target block.

When the number of channels in an image is 3, the value of cIdx in Code10 may be one of the values {0, 1, 2}. For example, the value of cIdxmay be one of the predefined values {0, 1, 2}.

At step 2010, the communication unit 1720 may receive a bitstream.

At step 2020, the processing unit 1710 may determine whether it ispossible to skip a transform for the target block.

When it is possible to skip a transform, step 2030 may be performed.

When it is not possible to skip a transform, step 2060 may be performed.

For example, the processing unit 1710 may determine that it is notpossible to skip a transform when the size of the target block is lessthan or equal to a specific size.

For example, the processing unit 1710 may determine that it is notpossible to skip a transform when the size of the target block isgreater than a specific size.

Here, the specific size may be a boundary value for the block size forwhich it is permitted to skip a transform.

For example, when the condition in the following Code 7 is satisfied(i.e., when the result of the condition in Code 7 is true), theprocessing unit 1710 may determine that it is not possible to skip atransform, whereas when the condition in the following Code 7 is notsatisfied (i.e. when the result of the condition in Code 7 is false),the processing unit 1710 may determine that it is possible to skip atransform.

if (log 2TrafoSize<=Log 2MaxTransformSkipSize)  [Code 11]

At step 2030, the processing unit 1710 may determine whether the sharingof transform_skip_flag information with the target channel of the targetblock is to be used based on the selected representative channel.

If it is determined that the transform_skip_flag information is not tobe shared, step 2040 may be performed.

If it is determined that the transform_skip_flag information is to beshared, step 2050 may be performed.

For example, when the condition in the following Code 8 is satisfied(i.e., when the result of the condition in Code 8 is true), theprocessing unit 1710 may determine that the transform_skip_flaginformation is to be shared, whereas when the condition in the followingCode 8 is not satisfied (i.e. when the result of the condition in Code 8is false), the processing unit 1710 may determine that thetransform_skip_flag information is not to be shared.

if ((cIdx !=SelectedCIDX)&& “cross-channel prediction is used”)  [Code12]

In other words, at step 2030, when the target channel is not a selectedrepresentative channel indicated by SelectedCIDX and cross-channelprediction is used for the target block, the processing unit 1710 maydetermine to share the transform_skip_flag information with the targetchannel. Further, when the target channel is the selected representativechannel indicated by SelectedCIDX or when cross-channel prediction isnot used for the target block, the processing unit 1710 may determinenot to share the transform_skip_flag information.

At step 2040, the processing unit 1710 may acquire thetransform_skip_flag information of the target channel from thebitstream. The processing unit 1710 may parse and read thetransform_skip_flag information of the target channel from thebitstream.

The transform_skip_flag information may be stored in transform_skip_flag[x0][y0][cIdx].

At step 2050, the processing unit 1710 may set the transform_skip_flaginformation so that the transform_skip_flag information of a selectedrepresentative channel indicated by SelectedCIDX is used as thetransform_skip_flag information of the target channel.

The processing unit 1710 may use the transform_skip_flag information ofthe selected representative channel, indicated by SelectedCIDX, as thetransform_skip_flag information of the target channel, instead ofparsing and reading the transform_skip_flag information of the targetchannel from the bitstream. In other words, the processing unit 1710 maystore the value of transform_skip_flag [x0][y0][SelectedCIDX] intransform_skip_flag [x0][y0][cIdx].

That is, the value previously stored intransform_skip_flag[x0][y0][SelectedCIDX] may also be equally used intransform_skip_flag[x0][y0][cIdx], without requiring a procedure forparsing and reading the transform_skip_flag information of the targetchannel from the bitstream.

At step 2060, information indicating that a transform is not to beskipped for the target channel of the target block may be set. Becauseskipping a transform for the target block is not permitted, the value oftransform_skip_flag [x0][y0][cIdx] may be set to 0.

In other words, for the target channel of the target block indicated bycIdx, a predefined value of 0 may be set in the transform_skip_flaginformation in order to indicate that a transform is not to be skippedfor the target channel, instead of parsing and readingtransform_skip_flag information indicating whether a transform is to beskipped from the bitstream.

As described above in the above-described embodiment, the codingdecision information of a selected representative channel may be sharedwith all channels except the selected representative channel.

The above-described embodiment may be partially modified. In otherwords, the coding decision information of the selected representativechannel may be shared as the coding decision information of anotherspecified channel. For example, the coding decision information mayinclude transform_skip_flag information.

For example, when the value of SelectedCIDX is 1, a cIDX value of 1indicates the Cb signal, and a cIDX value of 2 indicates the Cr signal,coding decision information for the Cb signal may be shared as thecoding decision information of the Cr signal.

In other words, the coding decision information for the Cb signal may betransmitted from the encoding apparatus 1600 to the decoding apparatus1700, and the coding decision information for the Cr signal may be setusing the coding decision information for the Cb signal, rather thanbeing separately transmitted.

When the coding decision information for the Cb signal is shared as thecoding decision information for the Cr signal, the above-described Code10 may be modified to the following Code 13:

    [Code 13]   if ( log2TrafoSize <= Log2MaxTransformSkipSize ) {   if(( cIdx != 2) or ( ! “cross-channel prediction is used” )) // !means logical negation    PARSE    transform_skip_flag[ x0 ][ y0 ][ cIdx] from compressed bitstream // step 2040   Else    transform_skip_flag[ x0  ][  y0  ][cIdx]  =   transform_skip_flag [x 0 ][ y0 ][ SelectedCIDX] // step 2050   } else    transform_skip_flag[ x0 ][ y0 ][cIdx] = 0 //step 2060

At step 2030, when the condition in the following Code 8 is satisfied(i.e. when the result of the condition in Code 8 is true), theprocessing unit 1710 may determine that the transform_skip_flaginformation is to be shared, whereas when the condition in the followingCode 8 is not satisfied (i.e. when the result of the condition in Code 8is false), the processing unit 1710 may determine that thetransform_skip_flag information is not to be shared.

if ((cIdx !=2) or (!“cross-channel prediction is used”))  [Code 14]

In other words, at step 2030, the processing unit 1710 may determine notto share transform_skip_flag information with the target channel 1) whenthe target channel is not a channel with which the coding decisioninformation of the selected representative channel indicated bySelectedCIDX is shared, or 2) when cross-channel prediction is not usedfor the target block. Also, the processing unit 1710 may determine toshare the transform_skip_flag information 1) when the target channel isa channel with which the coding decision information of the selectedrepresentative channel indicated by SelectedCIDX is shared, and 2) whencross-channel prediction is used for the target block.

The steps in Code 13 may be implemented as other steps at which the samemeaning is maintained. For example, Code 13 may be modified to thefollowing Code 15:

    [Code 15]   if ( log2TrafoSize <= Log2MaxTransformSkipSize ) {    if((cIdx == 2) && (“cross-channel prediction is used”))   transform_skip_flag[  x0  ][  y0  ][  cIdx  ] = transform_skip_flag [x0 ][ y0 ][ SelectedCIDX ] // step 2050    Else    PARSEtransform_skip_flag[ x0 ][ y0 ][ cIdx ] from compressed bitstream //step 2040   } else    transform_skip_flag[ x0 ][ y0 ][ cIdx ] = 0 //step 2060

Sharing of Transform Selection Information

FIG. 21 is a flowchart of a method for sharing transform selectioninformation according to an embodiment.

In the above-described embodiments, transform_skip_flag information hasbeen described as coding decision information to be shared. Thetransform_skip_flag information in the above-described embodiments maybe replaced with another type of coding decision information. Below,transform selection information will be described as coding decisioninformation to be shared.

The transform selection information may be information indicating whichtransform is to be used for a transform block of a target channel. Thetransform selection information may include the above-described primarytransform selection information and/or secondary transform selectioninformation.

There may be a considerable correlation between the luma channel (i.e.Y) channel and the chroma channel (i.e. Cb and/or Cr) of an image. Forexample, the luma channel may include a large amount of informationabout the texture of the image, and the Cb channel and the Cr channel,which are chroma channels, may additionally provide color information tobe added to the texture.

Therefore, when prediction required for compression and reconstructionof an image is performed, prediction values for the Cb block and the Crblock for which prediction is performed from the signal of the lumachannel, previously acquired through decoding, may be calculated insteadof independent predictions being respectively performed for threechannels of a color space. Since a considerable amount of textureinformation of a chroma signal may be included in a luma signal, suchcross-channel prediction may be effective.

The case where cross-channel prediction is selected from among varioustechnologies including angular prediction, DC prediction, planarprediction, etc. for predicting a chroma signal or where cross-channelprediction is more profitable may mean that the signal properties of aluma channel are considerably similar to those of a chroma channel (i.e.Cb and/or Cr).

Therefore, when it is profitable to use a specific transform amongmultiple transforms for the luma block, it may be profitable to use thesame transform for the chroma block (i.e. the Cb block and/or the Crblock).

In other words, generally, the same transform may be used for the lumablock and the chroma block. Alternatively, when a specific transform isused for the luma block, an additional specific transform correspondingto the specific transform used for the luma block may be used for thechroma block.

Therefore, when cross-channel prediction is used, if one transform isdetermined, the determined transform may be equally used for the lumachannel and chroma channel(s), i.e., three channels, instead oftransforms used for the luma channel and chroma channel(s) beingrespectively signaled.

Alternatively, if one transform is determined for the luma channel, atransform corresponding to the transform determined for the luma channelmay be used for the chroma channels.

When the transform to be used for channels is determined in this way,the luma channel and the chroma channel(s) may be encoded based on thedetermination. For such encoding, one of multiple available transformsmay be selected only for the luma channel, and the one transform for theluma channel is selected, and thus a transform for the chroma channel(s)may be automatically determined.

Alternatively, encoding using the same transform may be performed onthree channels. By means of this encoding, the rate-distortion valuesmay be calculated for multiple transforms. Thereafter, through acomparison between the rate-distortion values of applicable transforms,a transform having the most profitable rate-distortion value may beselected, and encoding may be performed depending on the selectedtransform.

Alternatively, encoding using a transform set may be performed on threechannels. The transform set may include a specific transform used forthe luma channel and transform(s) that are used for chroma channel(s)and correspond to the specific transform.

By means of this encoding, the rate-distortion values may be calculatedfor multiple transform sets. Thereafter, through a comparison betweenthe rate-distortion values of applicable transform sets, the transformset having the most profitable rate-distortion value may be selected,and encoding may be performed depending on the selected transform set.

In an embodiment, instead of separately signaling pieces of transformselection information for multiple channels, transform selectioninformation may be signaled only for a luma signal (luma channel), andtransform selection information may not be separately signaled for theremaining channels that are chroma channels.

In an embodiment, the decoding apparatus 1700 may acquire transformselection information from a bitstream.

At step 2110, a communication unit 1720 may receive the bitstream.

At step 2120, a processing unit 1710 may determine whether the sharingof transform selection information with a target channel of a targetblock is to be used.

When the transform selection information is not to be shared, step 2130may be performed.

When the transform selection information is to be shared, step 2140 maybe performed.

At step 2130, the processing unit 1710 may acquire the transformselection information of the target channel from the bitstream. Theprocessing unit 1710 may parse and read the transform selectioninformation of the target channel from the bitstream.

At step 2140, the processing unit 1710 may set the transform selectioninformation so that the transform selection information of arepresentative channel is used as the transform selection information ofthe target channel.

Steps 2120, 2130, and 2140 may be represented by the following Code 16:

    [Code 16]   if( (cIdx != 0) && “cross-channel prediction is used”)   transform selection information [ x0 ][ y0 ][ cIdx ] = transformselection information [x0 ][ y0 ][0]   else    PARSE transform selectioninformation [  x0  ][  y0  ][  cIdx  ] from compressed bitstream

x0 and y0 may be spatial coordinates indicating the location of thetarget block.

cIdx may indicate the target channel of the target block.

When the number of channels in an image is 3, the value of cIdx in Code10 may be one of the values {0, 1, 2}. For example, the value of cIdxmay be one of the predefined values {0, 1, 2}.

cIdx of the representative channel may be assumed to be 0.

cIdx !=0″ may indicate that the target channel is not a representativechannel (e.g. a luma channel). A cIdx value of 0 may indicate that thetarget channel is the representative channel.

In other words, at step 2120, when the target channel is not arepresentative channel and cross-channel prediction is used for thetarget block, the processing unit 1710 may determine that the sharing oftransform selection information with the target channel is to be used.When the target channel is a representative channel or whencross-channel prediction is not used for the target block, theprocessing unit 1710 may determine that the sharing of transformselection information with the target channel is not to be used.

At step 2130, the processing unit 1710 may acquire the transformselection information of the target channel from the bitstream. Theprocessing unit 1710 may parse and read the transform selectioninformation of the target channel from the bitstream.

The transform selection information may be stored in transform selectioninformation[x0][y0][cIdx].

At step 2140, the processing unit 1710 may set the transform selectioninformation so that the transform selection information of therepresentative channel is used as the transform selection information ofthe target channel.

The processing unit 1710 may use the transform selection information ofthe representative channel as the transform selection information of thetarget channel, instead of parsing and reading the transform selectioninformation of the target channel from the bitstream. That is, theprocessing unit 1710 may store the value of transform selectioninformation[x0][y0][0] in transform selection information [x0][y0][cIdx].

That is, without requiring a procedure for parsing and reading thetransform selection information of the target channel from thebitstream, the value previously stored in transform selectioninformation[x0][y0][0] may be equally used in transform selectioninformation[x0][y0][cIdx].

Determination of Whether Cross-Channel Prediction is to be Used

In the embodiments described above with reference to FIGS. 18 to 21, ithas been exemplified that whether cross-channel prediction is to be usedis determined by checking whether the intra-prediction mode of a targetblock is one of an INTRA_CCLM mode, an INTRA_MMLM mode, and anINTRA_MFLM mode. In other words, when the intra-prediction mode of thetarget block is one of the INTRA_CCLM mode, the INTRA_MMLM mode, and theINTRA_MFLM mode, cross-channel prediction may be used. When theintra-prediction mode of the target block is none of the INTRA_CCLMmode, the INTRA_MMLM mode, and the INTRA_MFLM mode, cross-channelprediction may not be used.

The above-described determination of whether cross-channel prediction isto be used is only an example, and whether cross-channel prediction isto be used may be determined through one of the following Codes 17 to23. For example, when the value of the condition in each of thefollowing Codes is true, cross-channel prediction may be used, whereaswhen the value of the condition in each of the following Codes is false,cross-channel prediction may not be used. “intra_chroma_pred_mode” maybe the intra-prediction mode of a chroma channel.

if (intra_chroma_pred_mode==CCLM mode)  [Code 17]

if (intra_chroma_pred_mode==DM mode)  [Code 18]

if (intra_chroma_pred_mode==INTRA_CCLM mode)  [Code 19]

if (intra_chroma_pred_mode==INTRA_MMLM mode)  [Code 20]

if (intra_chroma_pred_mode==INTRA_MFLM mode)  [Code 21]

if ((intra_chroma_pred_mode==INTRA_CCLMmode)∥(intra_chroma_pred_mode==INTRA_MMLMmode)∥(intra_chroma_pred_mode==INTRA_MFLM mode))  [Code 22]

if ((intra_chroma_pred_mode==DMmode)∥(intra_chroma_pred_mode==INTRA_CCLMmode)∥(intra_chroma_pred_mode==INTRA_MMLMmode)∥(intra_chroma_pred_mode==INTRA_MFLM mode))  [Code 23]

Each of the CCLM mode, the DM mode, the INTRA_CCLM mode, the INTRA_MMLMmode, and the INTRA_MFLM mode described in Codes 17 to 23 may indicateone value of intra_chroma_pred_mode presented in a first column in theabove-described Tables 10 and 11. In relation to the CCLM mode, the DMmode, the INTRA_CCLM mode, the INTRA_MMLM mode, and the INTRA_MFLM mode,the foregoing description made above in relation to Tables 10 and 11 maybe referred to.

When it is intended to determine whether cross-channel prediction is tobe used, the size of a block may be additionally considered in schemesin the above-described Codes 17 to 23.

The determination of whether cross-channel prediction is to be used mayalso be performed through one of the following Codes 24 to 30. Forexample, when the value of the condition in each of the following Codesis true, cross-channel prediction may be used, whereas when the value ofthe condition in each of the following Codes is false, cross-channelprediction may not be used.

if ((intra_chroma_pred_mode==CCLM mode)&& block size condition)  [Code24]

if ((intra_chroma_pred_mode==DM mode)&& block size condition)  [Code 25]

if ((intra_chroma_pred_mode==INTRA_CCLM mode)&& block sizecondition)  [Code 26]

if ((intra_chroma_pred_mode==INTRA_MMLM mode)&& block sizecondition)  [Code 27]

if ((intra_chroma_pred_mode==INTRA_MFLM mode)&& block sizecondition)  [Code 28]

if ((intra_chroma_pred_mode==INTRA_CCLMmode)∥(intra_chroma_pred_mode==INTRA_MMLMmode)∥(intra_chroma_pred_mode==INTRA_MFLM mode)&& block sizecondition)  [Code 29]

if ((intra_chroma_pred_mode==DM(directmode)mode)∥(intra_chroma_pred_mode==INTRA_CCLMmode)∥(intra_chroma_pred_mode==INTRA_MMLMmode)∥(intra_chroma_pred_mode==INTRA_MFLM mode)&& block sizecondition)  [Code 30]

The block size conditions presented in Code 24 to Code 30 may bereplaced with one of the following codes Code 31, Code 32, Code 33, andCode 34.

((log 2TbWidth<=Log 2MaxSizeWidth)&& (log 2TbHeight Log2MaxSizeHeight))  [Code 31]

((log 2TbWidth>=Log 2MinSizeWidth)&& (log 2TbHeight>=Log2MinSizeHeight))  [Code 32]

((log 2TbWidth>Log 2MinSizeWidth)&& (log 2TbHeight Log2MaxSizeHeight))  [Code 33]

((log 2TbWidth>Log 2MinSizeWidth)&& (log 2TbHeight Log2MaxSizeHeight))  [Code 34]

log 2TbWidth and log 2TbHeight have been described above with referenceto Equations 11 and 12.

Log 2MaxSizeWidth, Log 2MaxSizeHeight, Log 2MinSizeWidth and Log2MinSizeHeight may be predefined values. Log 2MaxSizeWidth may be thewidth of a block having the maximum size. Log 2MaxSizeHeight may be theheight of the block having the maximum size. Log 2MinSizeWidth may bethe width of a block having the minimum size. Log 2MinSizeHeight may bethe height of the block having the minimum size.

For example, the value of Log 2MaxSizeWidth may be 16, the value of Log2MaxSizeHeight may be 16, the value of Log 2MinSizeWidth may be 4, andthe value of Log 2MinSizeHeight may be 4.

Alternatively, the value of Log 2MaxSizeWidth may be 32, and the valueof Log 2MaxSizeHeight may be 32.

When a DM is used, the intra-prediction mode of a chroma signal may notbe separately signaled. When a DM is used, an intra-prediction modesignaled for a luma signal may also be used in a chroma mode withoutchange.

Encoding and Decoding Using Sharing of Selective Information BetweenChannels Under Block Partition Structure

Generally, when an image is encoded, suitable encoding schemes may beseparately used for multiple spatial areas in consideration of spatialcharacteristics in the image. For this encoding, an image may bepartitioned into CUs, and the CUs generated from partitioning may berespectively encoded.

In order to perform this encoding, the same block partition structuremay be used for a luma channel and for a chroma channel.

However, the characteristics of a luma signal and the characteristics ofa chroma signal may be different from each other. In consideration ofthe difference between the characteristics, different block partitionstructures may be respectively used for the luma channel and the chromachannel in order to achieve more effective encoding.

Hereinafter, the case where the block partition structure of the imageis identical for the luma signal and the chroma signal (or multiplechannels) is referred to as a “single-tree block partition structure” or“single tree”.

Hereinafter, the case where the block partition structure of the imageis not identical for the luma signal and the chroma signal (or multiplechannels) is referred to as a “dual-tree block partition structure” or“dual tree”.

In an embodiment, a block of an additional channel to which a targetblock of a target channel corresponds may be specified between a lumachannel and a chroma channel (or multiple channels). The block of theadditional channel corresponding to the target block of the targetchannel is referred to as a “corresponding block (col-block)”.

In an embodiment, when the luma channel and the chroma channel (ormultiple channels) have the same block partition structure (i.e. when asingle tree is applied), the block of an additional channel to which thetarget block of the target channel corresponds may be specified.

In an embodiment, when the luma channel and the chroma channel (ormultiple channels) have different block partition structures (i.e. whena dual tree is applied), the block of an additional channel to which thetarget block of the target channel corresponds may be specified.

When the luma channel and the chroma channel (or multiple channels) havedifferent block partition structures (i.e. when a dual tree is applied),coding decision information of the corresponding block may be sharedwith the target block of the target channel. Through the sharing of thecoding decision information, the target block is encoded, and thusencoding efficiency may be improved.

The coding decision information of the corresponding block, whichcorresponds to the target block of the target channel, may be parsedfrom a compressed bitstream.

The coding decision information of the corresponding block, whichcorresponds to the target block of the target channel, may be parsedfrom the compressed bitstream, and the target block of the targetchannel may be decoded using the coding decision information of thecorresponding block.

For example, transform_skip_flag information of the corresponding block,which corresponds to the target block of the target channel, may beparsed, and the transform_skip_flag information of the correspondingblock may be used to determine whether a transform is to be skipped forthe target block.

Shared information may be coding decision information that is sharedbetween the luma block and the chroma block.

When the block partition structure of a first channel and the blockpartition structure of a second channel are identical to each other, ifthe spatial location of the first block of the first channel correspondsto the spatial location of the second block of the second channel, thefirst block and the second block may correspond to each other (i.e. maybe co-located). In other words, the corresponding blocks of differentchannels may be blocks in different channels having corresponding(co-located) spatial locations. Shared information of the second block,corresponding to the first block, may be used for encoding and/ordecoding of the first block.

When the block partition structure of the chroma channel is identical tothe block partition structure of the luma channel, a luma blockcorresponding to a specific chroma block of the chroma channel may be aluma block at a spatial location corresponding to the spatial locationof the specific chroma block. In this case, shared information of theluma block corresponding to the chroma block may be used for encodingand/or decoding of the chroma block.

FIG. 22 illustrates a single-tree block partition structure.

FIG. 23 illustrates a dual-tree block partition structure.

Under a 4:2:0 color subsampling structure, a luma signal regionspatially corresponding to a chroma block may occupy an area four timesas large as the chroma block. In other words, the horizontal length(width) and the vertical length (height) of the luma signal region maybe twice the horizontal length and the vertical length of the chromablock.

As illustrated in FIG. 22, in the corresponding image regions of achroma channel and a luma channel, the block partition structure of thechroma channel and the block partition structure of the luma channel maybe identical to each other. In other words, a single tree may be usedfor the chroma channel and the luma channel.

In FIG. 22 and subsequent drawings, image regions corresponding to eachother are indicated by “corresponding regions”.

As illustrated in FIG. 23, in the corresponding image regions of thechroma channel and the luma channel, the block partition structure ofthe chroma channel and the block partition structure of the luma channelmay be different from each other. In other words, a dual tree may beused for the chroma channel and the luma channel.

For example, in FIG. 23, the region of a luma channel spatiallycorresponding to one chroma block may be partitioned into eight blocks.

When block partition structures in the corresponding image regions ofthe luma channel and the chroma channel are identical to each other, ablock of the luma channel spatially corresponding to a specific chromablock may be unambiguously specified.

In contrast, when the block partition structure of the luma channel isnot identical to the block partition structure of the chroma channel,for a specified chroma block determined through partitioning dependingon the block partition structure of the chroma channel, a luma blockcorresponding to the specified chroma block may not be definitelyspecified in the luma channel. As illustrated in FIG. 23, this ambiguityis due to the fact that the block partition structure of the chromachannel and the block partition structure of the luma channel are notidentical to each other.

In an embodiment, a method for specifying the luma block correspondingto the chroma block will be described for the case in which the blockpartition structure of the luma channel is not identical to the blockpartition structure of the chroma channel.

For example, multiple luma blocks corresponding to the chroma block maybe specified. By means of this specification, piece(s) of sharedinformation may be acquired from one or more luma blocks correspondingto the chroma block, which is a target block, and encoding and/ordecoding of the chroma block may be performed using the acquiredpiece(s) of shared information.

Below, a method for specifying one or more blocks of a second channelcorresponding to the block of a first channel will be described for thecase where the block partition structure of the first channel is notidentical to that of the second channel. In the following description,although the first channel will be described as being the chroma channeland the second channel will be described as being the luma channel, thechroma channel and the luma channel are merely exemplary, and the firstchannel and the second channel may be different types of channels, asdescribed above.

FIG. 24 illustrates a scheme for specifying a corresponding block basedon the location in a corresponding region according to an example.

A corresponding region indicating luma blocks corresponding to a chromablock may be specified as a rectangular region. The location of anuppermost-leftmost pixel in the rectangular region may be (xCb, yCb).The location of a lowermost-rightmost pixel in the rectangular regionmay be (xCb+cbWidth−1, yCb+cbHeight−1).

The location (xCb, yCb) may indicate the location of a luma pixelcorresponding to the location of an uppermost-leftmost pixel in thechroma block (i.e. chroma coding block).

cbWidth and cbHeight may be values respectively indicating the width andthe height of a target block based on the luma pixel.

In other words, the corresponding region indicating luma blockscorresponding to the chroma block may be defined as a rectangular regionin which the location of the uppermost-leftmost pixel based on thelocation of the luma pixel is (xCb, yCb), and which has a horizontalwidth of cbWidth and a vertical height of cbHeight.

The corresponding region indicating luma blocks corresponding to theabove-described chroma block may be applied to embodiments describedabove with reference to FIGS. 24 to 31.

In FIG. 24, the luma blocks corresponding to the chroma block may beluma blocks present at predefined locations in the corresponding regionof the luma channel spatially corresponding to the chroma block.

In other words, luma blocks present at predefined locations in thecorresponding region of the luma channel spatially corresponding to thechroma block may be specified as one or more luma blocks correspondingto the chroma block. Alternatively, luma blocks occupying predefinedlocations in the corresponding region of the luma channel spatiallycorresponding to the chroma block may be specified as one or more lumablocks corresponding to the chroma block.

For example, the predefined locations may be a center (CR) location, atop-left (TL) location, a top-right (TR) location, a bottom-left (BL)location, and a bottom right (BR) location in the region of the lumachannel spatially corresponding to the chroma block.

The CR location may indicate (xCb+cbWidth/2, yCb+cbHeight/2).

The TL location may indicate (xCb, yCb). The TR location may indicate(xCb+cbWidth−1, yCb). The BL location may indicate (xCb,yCb+cbHeight−1). The BR location may indicate (xCb+cbWidth−1,yCb+cbHeight−1).

In an embodiment, a block of an additional channel to which the targetblock of the target channel corresponds may be specified among multiplechannels, such as a luma channel and a chroma channel. The block of theadditional channel corresponding to the target block of the targetchannel is referred to as a “corresponding block”.

In an embodiment, in the region of the luma channel spatiallycorresponding to the chroma block, a luma block including a luma pixelpresent at the location (xCb+cbWidth/2, yCb+cbHeight/2) indicative ofthe center (CR) may be the corresponding block. Therefore, when thetarget block is partitioned in the form of a dual tree, a certain blockof an additional channel (e.g. a luma channel) to which the target blockof the target channel (e.g. a chroma channel) corresponds, amongmultiple channels, may be unambiguously specified. In an embodiment, inthe encoding and/or decoding of the chroma channel, a luma blockincluding a luma pixel present at the location (xCb+cbWidth/2,yCb+cbHeight/2) may be specified as the corresponding block. Informationabout the specified corresponding block may be used to encode and/ordecode the target block.

For example, the predefined locations may be some of the CR location, TLlocation, TR location, BL location, and BR location in the region of theluma channel spatially corresponding to the chroma block.

For example, the luma blocks corresponding to the chroma block may beblocks including at least one of pixels located at the center location,the top-left location, the top-right location, the bottom-left location,and the bottom-right location in the region of the luma channelspatially corresponding to the chroma block.

For example, the luma blocks corresponding to the chroma block may besome of blocks including at least one of pixels located at the centerlocation, the top-left location, the top-right location, the bottom-leftlocation, and the bottom-right location in the region of the lumachannel spatially corresponding to the chroma block.

For example, the luma blocks corresponding to the chroma block mayinclude blocks including at least one of pixels located at the centerlocation, the top-left location, the top-right location, the bottom-leftlocation, and the bottom-right location in the region of the lumachannel spatially corresponding to the chroma block.

FIG. 25 illustrates a scheme for specifying a corresponding block basedon the area in a corresponding region according to an example.

FIG. 26 illustrates another scheme for specifying a corresponding blockbased on the area in a corresponding region according to an example.

Luma blocks corresponding to a chroma block may indicate the luma blockhaving the largest area in the region of a luma channel spatiallycorresponding to the chroma block.

Alternatively, the luma blocks corresponding to the chroma block may bea predefined number of luma blocks having the largest area in the regionof the luma channel spatially corresponding to the chroma block.

Such a specifying scheme may be due to the fact that there is a strongpossibility that the features of the luma block having the largest areain the region of the luma channel corresponding to the chroma block or apredefined number of luma blocks having the largest area will be similarto those of the chroma block.

As illustrated in FIG. 25, the predefined number may be 2. In FIG. 25,two blocks (i.e. block 1 and block 2) having the largest area may beselected from among eight luma blocks in the region of the luma channelcorresponding to the chroma block.

As illustrated in FIG. 26, the predefined number may be 3. In FIG. 25,three blocks (i.e. block 1, block 2, and block 3) having the largestarea may be selected from among eight luma blocks in the region of theluma channel corresponding to the chroma block.

Through this specifying scheme, even if the block partition structuresof the luma channel and the chroma channel are not identical to eachother, encoding efficiency may be improved by utilizing sharedinformation.

FIG. 27 illustrates a scheme for specifying a corresponding block basedon the form of a block in a corresponding region according to anexample.

FIG. 28 illustrates another scheme for specifying a corresponding blockbased on the form of a block in a corresponding region according to anexample.

Luma blocks corresponding to a chroma block may indicate a luma block,having the same form as the chroma block, in the region of a lumachannel spatially corresponding to the chroma block.

For example, the form of the block may include the size of the block.

Depending on the dual-tree block partition structure, such as thoseillustrated in FIGS. 27 and 28, the block partition structure of a CU ofthe chroma channel may not be identical to the block partition structureof a CU in the region of the luma channel corresponding to the CU of thechroma channel. Even in this case, the region of the luma channelcorresponding to the region of a target block in the CU of the chromachannel may be present as a single block.

In this case, a single luma block in the region of the luma channelcorresponding to the chroma block may accurately match the chromachannel. Shared information of the matched luma block may be shared ascoding decision information of the chroma block.

FIG. 29 illustrates a scheme for specifying a corresponding block basedon the aspect ratio of a block in a corresponding region according to anexample.

FIG. 30 illustrates another scheme for specifying a corresponding blockbased on the aspect ratio of a block in a corresponding region accordingto an example.

Luma blocks corresponding to a chroma block may be luma block(s), havingthe same aspect ratio as the chroma block, in the region of a lumachannel spatially corresponding to the chroma block.

Alternatively, luma blocks corresponding to a chroma block may be lumablock(s), having aspect ratios similar to that of the chroma block, inthe region of the luma channel spatially corresponding to the chromablock.

For example, in the region of the luma channel of FIG. 29, luma block 2may be selected, and in the region of the luma channel of FIG. 30, lumablock 1 may be selected.

Here, the aspect ratio of the block may be the ratio of the horizontallength to the vertical length of the corresponding block. In otherwords, the aspect ratio of the block may be a value obtained by dividingthe horizontal length of the block by the vertical length of the block.

For example, whether the aspect ratios of blocks are equal to each othermay be determined using the following Equation 13:

(log₂Width_(Chroma)−log₂Height_(Chroma))==(log₂Width_(Luma)−log₂Height_(Luma))  [Equation13]

Width_(Chroma) may be the width of the chroma block. Height_(Chroma) maybe the height of the chroma block.

Width_(Luma) may be the width of the luma block corresponding to thechroma block. Height_(Luma) may be the height of the luma blockcorresponding to the chroma block.

Whether the aspect ratios are similar to each other may be determinedusing the following Equation 14:

|(log₂WidthChroma−log₂HeightChroma)==(log₂WidthLuma−log₂HeightLuma)|<THD  [Equation14]

“|x|” may indicate the absolute value of x.

THD may be a threshold value. For example, the value of THD may be 2.

According to Equations 13 and 14, one or more luma blocks having thesame aspect ratio as the chroma block may be specified as correspondingblocks. Alternatively, one or more luma blocks having aspect ratiossimilar to that of the chroma block may be specified as correspondingblocks.

FIG. 31 illustrates a scheme for specifying a corresponding block basedon the encoding properties of a block in a corresponding regionaccording to an example.

Although the block partition structure of a chroma channel and the blockpartition structure of a luma channel are independent of each other, ifa luma block having the same coding decision information as the chromablock is present among luma blocks in the region of the luma channelspatially corresponding to the chroma block, the shared information ofthe chroma block and the shared information of the luma block may beidentical to each other.

In order to utilize these characteristics, a luma block, having the samevalue as the chroma block for predefined coding decision information,may be specified as the luma block corresponding to the chroma block.Alternatively, a luma block, having a value similar to that of thechroma block for predefined coding decision information, may bespecified as the luma block corresponding to the chroma block.

For example, the predefined coding decision information may beinformation about whether intra-prediction is used, an intra-predictionmode, motion prediction information, a motion vector, information aboutwhether a merge mode is used, a derived mode, transform selectioninformation, etc.

For example, the intra-prediction mode may be used as predefined codingdecision information. In FIG. 31, luma block 1, luma block 2, and lumablock 3 are illustrated in the region of the luma channel. The lumablock 1, the luma block 2, and the luma block 3 may be luma blockshaving an intra-prediction mode identical to (or similar to) theintra-prediction mode of the chroma block. The luma block 1, the lumablock 2, and the luma block 3 may be specified as luma blockscorresponding to the chroma block.

In accordance with the above-described specific schemes, the luma blockcorresponding to the chroma block may include multiple luma blocks. Whenpieces of shared information of the multiple luma blocks are identicalto each other, no problem may occur when the pieces of sharedinformation are used for the chroma block. In contrast, when pieces ofshared information of multiple luma blocks are not identical to eachother, shared information to be used for encoding and/or decoding of thechroma block may be ambiguous.

A value corresponding to the majority of the values of pieces of sharedinformation of multiple corresponding blocks may be used as the value ofshared information of the chroma block. This decision method may bereferred to as a “majority-based shared information decision method”.Through this method, information to be shared may be efficiently sharedwithout additional signaling.

For example, when transform_skip_flag information is shared, a valueoccupying the majority of the values of pieces of transform_skip_flaginformation of multiple luma blocks corresponding to the chroma blockmay be shared as the value of the transform_skip_flag information of thechroma block. By means of this sharing, encoding and/or decoding of thechroma block may be performed.

For example, shared information may be used only when the number of lumablocks corresponding to the chroma block is only one. The sharedinformation may be used only when there is only one luma block thatsatisfies the above-described specific condition in the region of theluma channel spatially corresponding to the chroma block. Alternatively,when the region of an additional channel spatially corresponding to thetarget block of a target channel is specified only by a single block,coding decision information may be shared between channels.

Referring to FIG. 27, when the region of the luma channel spatiallycorresponding to the target block of the chroma channel is partitionedinto only one block, the one block may be the corresponding block of theluma channel. This shared information of the corresponding block may beused to encode and/or decode the chroma block.

When the region of the luma channel spatially corresponding to thetarget block of the chroma channel is not partitioned into one block,coding decision information may not be shared without separatesignaling.

FIG. 32 is a flowchart of an encoding method according to an embodiment.

The encoding method and a bitstream generation method according to thepresent embodiment may be performed by an encoding apparatus 1600. Theembodiment may be a part of a target encoding method or a video encodingmethod.

At step 3210, a processing unit 1610 may determine coding decisioninformation of the representative channel of a target block.

At step 3220, the processing unit 1610 may generate information aboutthe target block by performing encoding on the target block that usesthe coding decision information of the representative channel of thetarget block.

At step 3230, the processing unit 1610 may generate a bitstreamincluding the information about the target block.

The information about the target block may include coding decisioninformation of the representative channel. Also, the bitstream and theinformation about the target block may not include coding decisioninformation of a target channel.

The embodiment described with reference to FIG. 32 may be combined withthe above-described additional embodiments. Repetitive descriptions willbe omitted here.

FIG. 33 is a flowchart of a decoding method according to an embodiment.

The decoding method according to the present embodiment may be performedby a decoding apparatus 1600.

At step 3310, a communication unit 1710 may receive a bitstreamincluding information about a target block.

The information about the target block may include coding decisioninformation of a representative channel. Also, the bitstream and theinformation about the target block may not include coding decisioninformation of a target channel.

At step 3320, a processing unit 1710 may share the coding decisioninformation of the representative channel of the target block as thecoding decision information of the target channel of the target block.

The coding decision information of the representative channel may beshared as the coding decision information of the target channel.

At step 3230, the processing unit 1610 may perform decoding on thetarget block that uses the coding decision information of the targetchannel.

The embodiment described with reference to FIG. 33 may be combined withthe above-described additional embodiments. Repetitive descriptions willbe omitted here.

The above-described embodiments may be performed using the same methodin the encoding apparatus 1600 and the decoding apparatus 1700.

The sequences in which the steps, operations, and procedures are to beapplied in the embodiments may be different from each other in theencoding apparatus 1600 and the decoding apparatus 1700. Alternatively,the sequences in which the steps, operations, and procedures are to beapplied in the embodiments may be equal to each other in the encodingapparatus 1600 and the decoding apparatus 1700.

The embodiments may be respectively performed on a luma signal and achroma signal. Alternatively, the embodiments may be equally performedon the luma signal and the chroma signal.

The form of each block to which the embodiments are to be applied may bea square form or a non-square form.

Whether to apply the embodiments may be decided on based on the size ofat least one of a CU, a PU, a TU, and a target block. Here, the size maybe defined as the minimum size and/or the maximum size that enable theembodiments to be applied to the target, and may be defined as a fixedsize that enables the embodiments to be applied to the target.

Further, a first embodiment may be applied to a first size, and a secondembodiment may be applied to a second size. That is, the embodiments maybe complexly applied according to the size of the target. Also, theembodiments may be applied only to the case where the size of the targetis equal to or greater than the minimum size and is less than or equalto the maximum size. That is, the embodiments may be applied only to thecase where the size of the target falls within a certain range.

For example, the embodiments may be applied only to the case where thesize of the target block is equal to or greater than 8×8. For example,the embodiments may be applied only to the case where the size of thetarget block is 4×4. For example, the embodiments may be applied only tothe case where the size of the target block is less than or equal to16×16. For example, the embodiments may be applied only to the casewhere the size of the target block is equal to or greater than 16×16 andis less than or equal to 64×64.

Whether to apply the embodiments may be decided on depending on atemporal layer. In order to identify the temporal layers to whichembodiments are to be applied, separate identifiers may be signaled. Theembodiments may be selectively applied to temporal layers specified byidentifiers. Here, such an identifier may indicate the lowest layerand/or the highest layer to which the embodiments are to be applied, andmay also indicate a specific layer to which the embodiments are to beapplied. Further, a temporal layer to which the embodiments are to beapplied may be predefined.

For example, the embodiments may be applied only to the case where thetemporal layer of a target image is the lowest layer. For example, theembodiments may be applied only to the case where the temporal layeridentifier of the target image is equal to or greater than 1. Forexample, the embodiments may be applied only to the case where thetemporal layer of a target image is the highest layer.

The slice type to which the embodiments are to be applied may bedefined. Depending on the slice type, the embodiments may be selectivelyapplied.

In the above-described embodiments, although the methods have beendescribed based on flowcharts as a series of steps or units, the presentdisclosure is not limited to the sequence of the steps and some stepsmay be performed in a sequence different from that of the describedsteps or simultaneously with other steps. Further, those skilled in theart will understand that the steps shown in the flowchart are notexclusive and may further include other steps, or that one or more stepsin the flowchart may be deleted without departing from the scope of thedisclosure.

The above-described embodiments according to the present disclosure maybe implemented as a program that can be executed by various computermeans and may be recorded on a computer-readable storage medium. Thecomputer-readable storage medium may include program instructions, datafiles, and data structures, either solely or in combination. Programinstructions recorded on the storage medium may have been speciallydesigned and configured for the present disclosure, or may be known toor available to those who have ordinary knowledge in the field ofcomputer software.

A computer-readable storage medium may include information used in theembodiments of the present disclosure. For example, thecomputer-readable storage medium may include a bitstream, and thebitstream may contain the information described above in the embodimentsof the present disclosure.

The computer-readable storage medium may include a non-transitorycomputer-readable medium.

Examples of the computer-readable storage medium include all types ofhardware devices specially configured to record and execute programinstructions, such as magnetic media, such as a hard disk, a floppydisk, and magnetic tape, optical media, such as compact disk (CD)-ROMand a digital versatile disk (DVD), magneto-optical media, such as afloptical disk, ROM, RAM, and flash memory. Examples of the programinstructions include machine code, such as code created by a compiler,and high-level language code executable by a computer using aninterpreter. The hardware devices may be configured to operate as one ormore software modules in order to perform the operation of the presentdisclosure, and vice versa.

As described above, although the present disclosure has been describedbased on specific details such as detailed components and a limitednumber of embodiments and drawings, those are merely provided for easyunderstanding of the entire disclosure, the present disclosure is notlimited to those embodiments, and those skilled in the art will practicevarious changes and modifications from the above description.

Accordingly, it should be noted that the spirit of the presentembodiments is not limited to the above-described embodiments, and theaccompanying claims and equivalents and modifications thereof fallwithin the scope of the present disclosure.

1-20. (canceled)
 21. A decoding method, comprising: performing adetermination as to whether to perform decoding on at least three blocksusing a coding information shared for the at least three blocks; andperforming the decoding on the at least three blocks using the codinginformation based on the determination.
 22. The decoding method of claim21, wherein the decoding comprises intra predictions for the at leastthree blocks.
 23. The decoding method of claim 22, wherein the sharedcoding information comprises an intra prediction direction for the intrapredictions.
 24. The decoding method of claim 23, wherein sizes of theat least three blocks are the same.
 25. The decoding method of claim 21,wherein it is determined based on information acquired from a bitstreamwhether to perform the decoding for the at least three blocks using theshared coding information.
 26. The decoding method of claim 21, whereinit is determined based on a block size whether to perform the decodingon the at least three blocks using the shared coding information. 27.The decoding method of claim 21, wherein a Multiple Transform Selection(MTS) for a block is performed based on the determination.
 28. Thedecoding method of claim 21, wherein a transform for a verticaldirection and a transform for a horizontal direction for a block isperformed based on the determination.
 29. The decoding method of claim21, wherein a filtering for reference samples for the at least threeblock is performed based on the determination.
 30. The decoding methodof claim 21, wherein filterings for prediction blocks of the at leastthree blocks is performed based on the determination.
 31. The decodingmethod of claim 21, wherein channels of the at least three blocks aredifferent.
 32. The decoding method of claim 21, wherein the decodingcomprises in-loop filterings for the at least three blocks.
 33. Thedecoding method of claim 21, wherein the decoding comprisestransformations for the at least three blocks.
 34. The decoding methodof claim 21, wherein the decoding comprises a Cross Component LinerModel (CCLM) processing for the at least three blocks.
 35. The decodingmethod of claim 21, wherein information used for a reconstruction of ablock of one channel among the at least three blocks is used for areconstruction for a block of other channel among the at least threeblocks.
 36. The decoding method of claim 21, wherein the decodingcomprises a Multiple Transform Selection (MTS) for the at least threeblocks.
 37. The decoding method of claim 36, wherein a type of atransformation for the at least three blocks is selected based on thedetermination.
 38. An encoding method, comprising: generatinginformation for at least three blocks by performing encoding for the atleast three blocks; wherein the at least three blocks are encoded usingcoding information commonly applied to the at least three blocks.
 39. Anon-transitory computer-readable medium storing a bitstream generated bythe encoding method of claim
 38. 40. A non-transitory computer-readablemedium storing a bitstream, the bitstream comprising: information for atleast three blocks; wherein decoding for the at least three blocks isperformed using the information for the at least three blocks, adetermination as to whether to perform the decoding on the at leastthree blocks using a coding information shared for the at least threeblocks, and the decoding on the at least three blocks using the codinginformation is performed based on the determination.